Add support to stepcvt_export_all to limit the parts exported and support custom rotations

[sree314]

Currently stepcvt_export_all exports all parts in the STEP file according to an arbitrary rotation.

This is an experiment to see if a lightweight CLI-based STL exporter can be built.

Add support for a command line option --partid that lists parts to be exported:

stepcvt_export_all.py file.step --partid part1 --partid part2

should only export parts with those partid.

Also add support for command line option --rot that can apply custom rotations to a part:

stepcvt_export_all.py file.step --partid part1 --rot 0,45,0

will export only partid part1 with rotation around Y-axis of 45 degrees.

You may also want to include a command-line option --exclude to export all objects except those specified by --exclude

stepcvt_export_all.py file.step --exclude part2