Add disappear cost

[bentaculum]

This mirrors the Appear cost and the NodeAppear variable.

[codecov]

Codecov Report

Merging #52 (93f17cf) into main (842d15d) will increase coverage by 0.38%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
+ Coverage   92.47%   92.86%   +0.38%     
==========================================
  Files          30       32       +2     
  Lines         718      757      +39     
==========================================
+ Hits          664      703      +39     
  Misses         54       54              

Files Changed	Coverage Δ
motile/costs/__init__.py	100.00% <100.00%> (ø)
motile/costs/disappear.py	100.00% <100.00%> (ø)
motile/variables/__init__.py	100.00% <100.00%> (ø)
motile/variables/node_disappear.py	100.00% <100.00%> (ø)

[funkey]

Thanks a lot!