Imperial units support, improvements, fixes, plugins API expansion

[pierotofy]

Various improvements and fixes.

[pierotofy]

Well, I started adding support for imperial units (both international and US), because it's overdue.

[pierotofy]

#1065

[smathermather]

The Mars Climate Orbiter will thank you.

[pierotofy]

Also ~25% speed-up in rendering DSM/DTM tiles (when shaded).

[pierotofy]

This should be good to go. There's some improvements that could be done on the measurement plugin, such as the ability to update existing measurements when switching units, currently one needs to create new measurements to use a new unit system, but these can be addressed in the future...

Potree also has no support for US imperial units, so there's not exactly a 1-to-1 relation between 3D view and map view if the user chooses to use US imperial units (which they shouldn't, it's 2024).

[smathermather]

Is it too late to ask for a couple more imperial units, specifically the following length and area measures?:

Area: gallons per fathom (GPF)

Length: hogsheads per rood (HPR)

ht @ivangayton

[pierotofy]

Shouldn't be a problem, one can just modify Units.js and UnitSelector.jsx.

[smathermather]

Shouldn't be a problem, one can just modify Units.js and UnitSelector.jsx.

Super tempted to do that on crankyserver...