allow to specify units for STL output

[jmairboeck]

Currently, no explicit units can be specified for the resulting STL output. According to this OpenCASCADE forum post, OpenCASCADE converts STEP files to internal units when reading them (by default millimeters), so the STL output, as well as the unit for linear deflection is presumably also in this internal unit.

OCCT recognizes units used in the STEP file (which can be different for different sub-shapes) and converts all geometry to OCCT units (millimeters by default).

Sometimes STL files in a different unit would be beneficial. Maybe an option could be added to STEPToMesh to switch to a different unit before reading and converting the file, if this is supported by OpenCASCADE, e.g. -u inch or --unit inch for inches instead of millimeters. The default should remain millimeters (i.e. not change anything).