Add Plummer potential

[jobovy]

To do items:

• Implement the potential, its forces, and derivatives
• C implementation
• Documentation
• Add to changelog
• Add nemo functions and list somewhere in docs
• Fully test:
  • Derivatives (automatic)
  • C integration (automatic)
  • Actions
  • NEMO

[codecov-io]

Current coverage is 99.66%

Merging #242 into dev-v1.1 will change coverage by 0% by 006e0bd

Coverage Diff

@@            dev-v1.1    #242   diff @@
========================================
  Files             97      99     +2
  Stmts          14713   14796    +83
  Branches           0       0       
  Methods            0       0       
========================================
+ Hit            14663   14746    +83
  Partial            0       0       
  Missed            50      50       

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