Support/ Scale Arbitrary GDS Units

[dan-fritchman]

Thus far we have the tacit assumption that importable GDSII's have one of a handful of values for their GdsUnits: 1nm, 1µm, etc.
In practice this has generally been the case. Until it ain't. And on some recently encountered examples, it ain't.
Layout21's internal units can retain the relatively coarse order-of magnitude setting, so long as we scale GDS coordinates, probably around here:

Layout21/layout21raw/src/gds.rs

Line 564 in ef6a881

fn import_units(&mut self, units: &gds21::GdsUnits) -> LayoutResult<Units> {