Remove '_Simbody' from gait model names

[jimmyDunne]

I would like to remove the _simbody from gait2392 and gait2354. I'm sure there was a reasonable reason to differentiate them way back when, but now they are inconsistent from every other model— every model is a Simbody model.

This change would involve converting the (i) names of the models in the repo, (ii) setup files, and (iii) the Cmake lists. Externally I would update the documentation with the improved, shorter name.

I am looking for comment before I make a branch and do the conversions.

[tkuchida]

I agree with removing the "_simbody" suffix.

[chrisdembia]

I agree.

[jimmyDunne]

PR #79 has dealt with this.