Kicad StepUp STEP to MCAD exporter: Latest version, Faster! support for Env_Var and Alias

[maui]

@Shack
work in progress to add resolver ability for a second 3D path as ENV VAR…
I left some room in my ini file
[prefix3D] prefix3d_1 =
for a
[prefix3D] prefix3d_1 = prefix3d_2 =
(but still considering the simplest way the best)
Maurice