Import Basis Curves split shapes

**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

Start by reproducing the listed import/export steps with a USD BasisCurve containing several strings, then trace the mergeTransformAndShape handling for imported Maya curve shapes. Compare the exported USD structure with the expected single prim, and consider the proposed BasisCurve attribute or multi-shape merge behavior as the completion criteria.

Written by the indexing model from the issue text.