Pygran-analysis

Added
- Versioneer for automated version control
- Test cases for pytest
- Method __len__ for System class

Changed
- Refactored pakcage structure
- Fixed bug with computeIntensitySegregation: 'self.types' typo
- Method Particles.computeRDF no longer uses deprecated 'normed' in numpy.histogram
- Arg 'Npts' renamed to 'npts' in Method Particles.computeScaleSegregation

Removed
- Old versioning system
- Optimized core.pyx
- System.keys (undefined)

Added
- Submodule 'generator' from PyGran.simulation
- Support for VTK file formats: vtp + vtu

Changed
- Method computeRDF() in core.py*: added optional 'npts' arg
- Function hcp() in generator.py: added optional 'units' arg

Removed
- Method computeGCOM() in core.py*
- Attribute 'length' for Particles.molecule

Added

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