v0.2.0 release

This is the release note of v0.2.0.

Highlights

• Add PFP citation in README.md #2
• Use pymatgen for pbc neighbor search speed up #3

Bug fixes

• Fixed a bug for inputs with 0 adjacencies. #6 (Thank you @AkihideHayashi )
• Remove RuntimeError on no-cupy environment #8 (Thank you @So-Takamoto )
• Bugfix: batch calculation with abc=True #9 (Thank you @So-Takamoto )

Others

• move pysen to develop dependency #10 (Thank you @So-Takamoto )