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: https://github.com/dan-fritchman/Layout21/blob/ef6a881bb0ef9bbb4e4d214184f7a4d095a666db/layout21raw/src/gds.rs#L564