Update matfaust.Faust.

Fix Faust ctor:
- forgotten dtype copy when a Faust is in first argument.
- typo str instead of int2str.

Fix Faust.cat:
- keeping the same device as first Faust for the output Faust.

Fix Faust.plus:
- calling eye with the proper dtype and device parameters.

Fix Faust.factors:
- when returning a Faust it must be on the same device as the source Faust.

Minor changes:
- hiding output in Faust.device.