Pynucastro

* Improvements for locating rate files in directories also
containing python scripts or Jupyter notebooks

* Fixed a warning when using Numba

* Updates to the StarKiller Microphysics format for compile time
variables

* Enhancements for the RateCollection Explorer to use NetworkX 2.5

* Updated the requirements

* Added general support for tabulated weak rates from Suzuki, et
al. 2016

* Added a simple pp network example

* Updated StarKiller Microphysics output to track latest changes in
Microphysics network API

* Added a core developers policy in the Readme

* Replace double precision reals in StarKiller networks with custom
real type `_rt`

* Incorporate modifications to thermal neutrino Jacobian
contributions from Microphysics (210) in StarKiller networks

* Simplify rate evaluation code in StarKiller networks

* Deprecated standalone Fortran output with VODE integration.

* BaseFortranNetwork is now an abstract class to require users to
either use StarKillerNetwork or provide templates

* Fix tabular rate bug where electron chemical potential
contributions were not included

* Update documentation and code comments

* Add Numba support for Python networks

* Enable sparse Jacobian (CSR) for StarKiller networks

* Incorporate CUDA Fortran port for StarKiller networks including
tabulated rates

* Optimize rate screening evaluation for StarKiller networks

* Fix bug to include the electron fraction and correct density
dependence for Reaclib electron capture rates

* Allow a nuclide to be both reactant and product

* Updates an error message for multiple degenerate rates

* Add example code generating a 160-isotope network

* Fix table compilation issue for StarKiller networks

* Moved energy generation subroutine into RHS module for StarKiller
networks

* pynucastro published on JOSS is archived to Zenodo

* JOSS reviewer changes merged and pynucastro accepted to JOSS