Nestpy

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Update to nestpy internals, adding in basic LArNEST bindings
* The copy/paste method for the NEST bindings have been replaced with adding NEST and other modules (such as gcem and pybind11) as git submodules which are downloaded at compile time. LXe notebooks and python scripts have been moved to tutorials/arxiv, and new notebooks will be placed in the tutorials folder as they are created in the future. LAr functionality through LArNEST has been added and will be expanded upon in future releases.

* Suggestions/issues/errors should be added as github issues.

* Users may have to do before installing:
** `pip uninstall nestpy`

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nestpy specific code quality:
* Thanks to the great help of Joran Angevaare, we now use GitHub workflows and no longer use Travis for releases and testing.
* `pip install nestpy` works again.
* Should one want to recompile from source, you will still need to use `git clone` (for now).

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**Development version, not installable**
* No new physics or implementation beyond developing a stable release workflow.
* There will not be a release associated with this version since we wanted to test the entire workflow. The stable release one should use to incorporate changes from 2022-02-09 (`NEST v2.3.5`) is `nestpy v1.5.4.`

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New Physics:
* Perfectly vertical MIP tracks now work, and use latest beta model (Greg Rischbieter, LZ/UAlbany)
* Field in G4 in any direction not just vertical but e.g. radial OK, ala (n)EXO and PETALO (Paola Ferrario)

Code Quality:
* NEST: Geant4.9.11 & C++17 compatibility achieved (Paola Ferrario, PETALO/Basque Foundation for Science)
* Multiple scatter code warning addressed: unused variable (Greg Rischbieter, LZ/UAlbany)

nestpy Specific:
* N/A

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New Physics:
* dE/dx-based yield code moved (execNEST->NEST.cpp) for accessibility. Muons, MIPs, LIPs; random positions
* Initial or average dE/dx allowed, and use of ESTAR or custom power law, with variation around a mean dE/dx
* loopNEST for ER restored, with 1st-principles mod TIB model of recombination parameters for sustainability
* New multiple scatter tool allows for creation of 2+ ER-like/NR-like scatters, or mixed for inelastic, Migdal, etc.

Code Quality and/or Miscellaneous Bug Fixes:
* random exponential smarter sampling for small ranges especially for Kr83m times (Scott Kravitz, LZ/LBNL)
* D-D energy spectrum user-settable, serving as example for any NR calibrations (Greg Rischbieter, LZ/UAlbany)
* New truncated Gauss option, w/ truncation at 0 in 1st usage to solve S2 corner case (Scott Kravitz, LZ/LBNL)

nestpy Specific:
* N/A