Introduce 'to_numpy' / 'from_numpy' methods

[sdrave]

This implements the proposal in #252.

[sdrave]

@renemilk, we have again unsuccessful checks due to decreasing coverage :(

[pmli]

I guess 0.03% decrease in coverage is not that significant.

[renefritze]

I've deleted the codecov comment in the hope that it'll pop up with 'right' layout in a bit. What I don't understand atm is why #447 was apparently using the correct yaml, where here we got both the wrong comment layout and the disabled check.

[codecov]

Codecov Report

Merging #446 into master will increase coverage by 0.36%.
The diff coverage is 69.16%.

Impacted Files	Coverage Δ
src/pymor/grids/oned.py	98.11% <ø> (ø)	⬆️
src/pymor/operators/interfaces.py	68.42% <ø> (ø)	⬆️
src/pymor/grids/tria.py	98.38% <ø> (ø)	⬆️
src/pymor/grids/rect.py	93.27% <ø> (ø)	⬆️
src/pymor/grids/unstructured.py	95.91% <ø> (ø)	⬆️
src/pymor/gui/jupyter.py	9.83% <0%> (ø)	⬆️
src/pymor/gui/qt.py	24.64% <0%> (ø)	⬆️
src/pymor/discretizers/fv.py	84% <0%> (ø)	⬆️
src/pymor/reductors/residual.py	71.89% <0%> (ø)	⬆️
src/pymor/algorithms/to_matrix.py	100% <100%> (ø)	⬆️
... and 20 more

[renefritze]

@sdrave look who's back and again not using the same config as in that other PR

[sdrave]

@pmli, I would suggest that you merge sys-mor with master soonish s.t. these interface changes propagate to your branch ..