avoid use of StdVec in the python API

[nim65s]

Hi,

According to loco-3d/crocoddyl#795 (comment), we can avoid teh use of StdVec in the python API. This would be much cleaner for the end user.

It is used eg. in

tsid/exercizes/notebooks/TSID_ex1.ipynb

Line 330 in 567982b

"vector = pin.StdVec_StdString()\n",

[andreadelprete]

Good point. Just fixed with commit 7d240c3

[nim65s]

Thanks !