Danton

This release allows configuring the physics model for tau energy losses and for neutrino DIS interactions. In addition, it patches some bugs, and it adds Python3 compatibility.

- The physics model can now be configured using the `danton_physics_set` API function, or the corresponding data card option. For DIS, the user can switch between CSMS ([Cooper-Sarkar, Mertsch and Sarkar](https://arxiv.org/abs/1106.3723)), BGR18 ([Bertone, Gold and Rojo (2018)](https://arxiv.org/abs/1808.02034)) or LO (Leading order computation, following [Gandhi et al., (1995)](https://arxiv.org/abs/hep-ph/9512364)). By default, BGR18 is used. Note that in previous versions of DANTON, the LO computation was used . Note also that, with this new version, the kinematic is still simulated using LO expressions, even when using CSMS or BGR18 cross-sections. Only the total cross-section is rescaled. Corresponding PDFs are used, however, depending on the selected DIS physics model.

- The "requested" counter has been patched. Simulations should deliver a proper number of events, as requested by the user.

- The forward flux end condition has been patched. In forward mode, all particles should now properly stop when reaching the sampling surface. In previous versions, in some rare cases, secondary taus could stop before reaching the sampling surface.

- The recorder "verbosity" option has been removed, since it was not implemented, actually.

- The generation weight of forward Monte Carlo events has been patched. Forward and backward cases now deliver consistent weights. Previously, the forward Monte Carlo weight did not include the generation solid angle.

- The `danton.py` utility module now supports both Python 2 and 3.

This release patches a bug in the polarisation of tau-, modifying their decays.

- Previously both tau- and tau+ were assumed to be right-handed at decay, considering that they are produced in UHE DIS events. Though UHE leptons produced in such events are indeed ~100% longitudinaly polarized, the sign was wrong for tau-. This has been corrected. Now, tau- are assumed to be left-handed and tau+ are assumed to be right-handed.

This release updates the core dependencies of DANTON, namely PUMAS (v1.1), TURTLE (v0.10) and ALOUETTE (d55a88d). In addition, it patches some bugs in forward mode and adds the following features and API modifications:

- Geoid undulations are now supported using the EMG96 model, by default. Please check the documentation of the `danton_earth_update` function for more details.
- The `stack_size` option has been removed from the `danton_earth_update` function. If a tiled GDEM is used, then there is no soft limit on the number of concurrent tiles in memory.
- The *topography* option in data cards has been simplified. It now accepts a path to a GDEM or a number. In the latter case, the number value specifies the constant altitude of the topography.
- The `danton_run` function has been renamed to `danton_context_run` since it is a method of a simulation context.
- A single random stream is now used per simulation context, even when using TAUOLA. The seed can be set explicitly with the `danton_context_random_set` function, and it can be retrieved with the `danton_context_seed` function. A *seed* option was added to data cards. If `null`, which is the default, then the seed is initialised from the OS entropy, using `/dev/urandom`.
- The `danton_initialise` function has been simplified. In addition to the *lock* and *unlock* callbacks, it now takes a single extra argument specifying the *prefix* for physics data needed by DANTON.

This new release patches several bugs of DANTON v0.2 and adds support for OSX. In addition, the following improvements have been made:

* The `danton_initialise` function now allows to configure the data set to use for PUMAS
* The default data sets can be modified at compilation time by editing the Makefile (DANTON_DEFAULT_ variables).
* The dependencies to `libtiff` and `libpng` (TURTLE) can be disabled in the Makefile.
* The DANTON random engine and topography can be accessed with the new `danton_get` functions.

* Add an option to request a given number of events to be logged
* Add the WGS84 geodesics as a configurable option. Default is PREM with a spherical Earth.
* Add support for a detailed topography with [TURTLE](https://github.com/niess/turtle), reading from ASTER-GDEM2 tiles.
* Update [ENT](https://github.com/niess/ent) to the CT14-NLO PDF.
* Add custom run actions to the API and an optional dump of major Monte-Carlo steps, configurable from the JSON.

**Examples** of the new options can be found in [backward-tau-decays.json](share/cards/backward-tau-decays.json).