Underworld

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New:
* MOVE TO PYTHON3. PYTHON2 NO LONGER SUPPORTED.
* New Examples & tests.
* New Semi-Lagrangian Crank Nicolson advection diffusion scheme.
* Collective H5 read/write for faster parallel IO operations.
* Arbitrary coordinate `evaluate()` for swarm data using nearest
neighbouring (kdtree) algorithm. See function section of user guide.
* XDMF for subMesh element types.
* Async test runner.

Enhancements:
* Move all mpi functionality into `uw.mpi` submodule.
* Non-parallel h5py is now sufficient for UW, although
parallel h5py is recommended for larger parallel simulations.
* Integral returns numpy array results (as opposed to Py natives).
* `GlobalSpaceFillerLayout` deprecated.
* Other tweaks & bug fixes.

Documentation:
* Many class docstrings updates.
* Function user guide section updated.
* Improved error messaging in parallel.
* Stampede2 build & usage instructions.
* Update for NCI/Raijin build instructions.

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Documentation:
* Dockerfile for TACC/Stampede2
* New build instructions for NCI/Raijin.
* Add dev Binder to README.md

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New functionality:
* Timing module for high level profiling.
* Scaling module for numerical value scaling.
* Index set objects can now directly index numpy arrays.
* `show_grid()` image rendering.

Changes:
* glucifer Surface defaults to use onMesh=True.
* added `data` handle to swarm as shortcut to particle coordinates data.
* Adding aliases for mesh vertex sets (left,right,etc)
* Removed `unsupported` subrepo.
* Docker images switch to MPICH for Shifter compatibility at magnus.

Documentation:
* Examples reorganised.
* All examples now include quantitative tests.
* Moved many examples to models library repo.
https://github.com/underworldcode/model_library
* Added new example, 08_Uplift_TractionBCs.ipynb.
* Moved benchmarks and tutorials to models library repo.
* Relocated defunct models:
https://github.com/underworldcode/model_graveyard
* Big overhaul of user guide.
* User guide rendered to html for Readthedocs page.
* Updates for docstrings and doctests.
* Improved error messaging.

Fixes:
* Fix incorrect particle outside domain issue 335
* Other stability improvements.

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* Remove `warning` module visibility from top level underworld
module.

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Enhancements:
* Removed submodules in favour of direct access.
* Petsc-3.9.x compatible.
* Support for nested SafeMaths operations.
* Improved function error messaging.

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Documentation:
* Fixes for Sphinx API documentation generation.