Support different colormaps in GLPatchWidget

[sdrave]

This is some work I have done a while ago. We still need to wire everything up ...

[landscape-bot]

Repository health increased by 4% when pulling 6c121ec on colormap into 98b014e on master.

• 639 new problems were found (including 240 errors and 189 code smells).
• 1098 problems were fixed (including 19 errors and 421 code smells).

[sdrave]

I have now finished the colormap support and defaulted the new colormap to viridis. Please, everyone, have a look if the new code works correctly on your machine.

[ftalbrecht]

thermalblock_gui.py seems to work as expected under arch linux. Anything else I should test?

[sdrave]

Since thermalblock_gui.py is special case, better also have a look at thermalbock.py and burgers.py.

[ftalbrecht]

./thermalblock.py --plot-err --plot-solutions --plot-error-sequence ... and ./burgers.py ... work for me.

[renefritze]

All three look good for me

[pmli]

(If you're happy, I'm happy.)