Ecell4

- Add simulators for the enhanced Green's Function Reaction Dynamics and spatial Gillespie methods.
- new build system (moved from waf to CMake and Python wheel).
- new tutorial IPython notebooks.
- inline javascript particle visualizer in IPython notebook.
- Support a network-frees simulation using C++ rulebased modeling engine.

ecell4_base 2.1.0 only works with ecell4 1.2.1 and above

This release uses
- cmake 3.17.0
- gsl 2.6
- hdf5 1.10.5
- boost 1.73.0

New features
- OffLattice support (453)

Bug fixes and minor improvements
- Add Python wrapper for UnitSpecies APIs (454)
- 250 Make observers picklable (450)
- bug fixes and stability improvements in sGFRD (432)
- Add a package for Python3.8

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