Import Basis Curves split shapes

[vcandelam]

Is your feature request related to a problem? Please describe.
When importing a usd with a basis curve that has several strings it gets splitted into diferent shapes under the same maya transform. This isn't an issue by itself, but when exporting back to usd instead of merging into a single prim when using the "mergeTransformAndShape" flag it is defined as a prim per shape.

Describe the solution you'd like
I guess the import splitting the shapes is a limitation in mayas bezier curve definition, but maybe adding an attribute like the USD_typeName='BasisCurve' that can understands that the shapes under it must be merged together

Describe alternatives you've considered
Maybe expanding the "mergeTransformAndShape" to multiple shapes? I guess this is a limitation due tu merging two polygonal meshes but for curves it shouldn't be an issue

Additional context
Step to reproduce:
1- Create a usd with a basis curve with several strings (in bezier basis, is the only one I could import back to maya)
2- Import into maya
3- Export back

[maya-usd-git-sync]

Issue synced internally to EMSUSD-835