Modify to disable base params computation menu option

Modify to disable base inertial  params computation menu option for
non-fixed base robots since the base inertial parameters algorithm
doesn't provide support for non-fixed base robots.
Modify `Save As` feature

Modify `Save As` feature to prompt for a new robot name and a file
dialog box in the case a name isn't provided.
Add `skew2vec()` function
Modify composite inertia computation

Modify composite inertia computation to make sure that simplified
intermediate variables are used in all places.