Pygran-sim

Added
- Generic EngineAPI class for engines
- Simple engine for 1D contact mechanics
- Generic ProtoInput base class for engine params
- Methods EngineAPI.SetupPrint and EngineAPI.setupWrite
- Support for LIGGGHTS v3.8.0 and latest (86544c3)

Changed
- Refactored code for generic engine implementation
- Merged DEMPy and Liggghts into `LiggghtsAPI`, a subclass of EngineAPI
- Improved mechanism for runtime discovery of engines
- Fixed bug in DEM class setting total number of procs to 0 for single-core runs
- Lower-case package name

Removed
- Redundant engine_liggghts.DEMPy
- Methods EngineAPI.printSetup and EngineAPI.writeSetup

Added
- requiremens.txt to src dir

Changed
- Numerous minor fixes and improvements in code quality

Added
- Keyword 'psd_style' (default: 'numberbased') to particle distribution
- Versioneer for automated version control

Changed
- Log filename from 'dem' to 'pygran'
- Fixed bug (missing dimension for cylindrical boxes) in 'models' module
- CI migration to github actions

Removed
- generator module

Added

- Version printed to log file

Added

- Source code from PyGran repo
- CHANGELOG for version logging
- CircleCI config for continuous integration
- Added License file