Pystog

* Deprecated pandas dependency by using numpy arrays for storage (thanks to mdoucet!)
* Deprecated plotting capabilities and matplotlib dependency
* Deprecated Python 2 support (via removing __future__ imports)
* Deprecated the Transformer CLI tool (not the Transformer module, though)
* Small fixes to references of the repository before the migration to neutrons
* Renamed variables to remove the `df_` prefix since we not longer use pandas DataFrames.
* Added header section to notebooks to install local pystog and matplotlib via pip

* Change CI/CD from Travis -> GitHub Actions
* Added conda recipe
* Added Pipenv capability
* Removed Transformer CLI
* Moved PyStoG CLI and various bug fixes
* Added LAMMPS files to create test data
* Added calculation of uncertainties (see PR 23)
* Updates to test suite based on adding uncertainties
* Formatting changes (max line length from 100 -> 80)
* Updates to requirements files (added a separate dev requirements file)
* Updates to setup.py for using requirements files and update to authors list
* Fixed tox setup
* Updates to the tutorial notebooks

* Bug fix for setting r range from correct Pandas dataframe

* Bug fix for missing r range when no fourier filter applied

* Bug fix to CLI for missing setting Q if no fourier filter applied

* Added `_safe_divide` function to Converter class
* Removed `_extend_axis_to_low_end` in Transformer class
* Various bug fixes