This patch updates Cura and all associated dependencies to 4.7.1.

Also, it adds math/py-numpy-stl - an optional dependency that makes
loading STL files much faster. Without it, Cura was complaining:

2020-09-20 02:19:31,877 - WARNING - [MainThread] STLReader.STLReader.<module> [32]: Could not find numpy-stl, falling back to slower code.

I'm not sure where the 6.x version in cad/fdm_materials came from, seems
like now that project has its version in sync with the Cura version,
so I chose to increment PORTEPOCH for that. [Greg]

• fdm_materials adds some new materials usable with Cura
• I had to bump PORTEPOCH on a couple of ports
• py-numpy-stl was added on a separate commit
• Bump version checking in Cura on some dependancies

[db]

PR: ports/249468
Submitted by: greg@unrelenting.technology