Fix typos in wrappers of dartpy Skeletons

[chhinze]

getGravityForces() and getExternalForces() are fixed in
dartpy.dynamics.MetaSkeleton and dartpy.dynamics.Skeleton.
The wrong functions were called.

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Before creating a pull request

• Document new methods and classes
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• Set version target by selecting a milestone on the right side
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[codecov]

Codecov Report

Merging #1392 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1392   +/-   ##
=======================================
  Coverage   57.27%   57.27%           
=======================================
  Files         366      366           
  Lines       27432    27432           
=======================================
  Hits        15711    15711           
  Misses      11721    11721

[jslee02]

Probably these are replication errors. Good catch!