v1.6.0b2

This beta release introduces the new manage_mc_materials submodule for customization of the MC materials database distributed with PHITS Tools, meaning you can now add your own materials to the database for easier access / managing your own collection of PHITS/MCNP-formatted materials. The following changes have been made to PHITS-Tools/MC_materials/

• The manage_mc_materials.py submodule has been added (for more details, see its documentation preview).
• Distributed pickle files have been replaced with JSON files (code updated accordingly).

Additionally, the pyproject.toml file used for building the distribution is now included. Note that within it and .gitattributes are specified files which appear in the repository for reference purposes but are not included in the distributed versions since they can be recreated locally (or just downloaded from the repository) and to keep the download size smaller.