Add flux gate to numerical model for mass-conservation tests

[fmaussion]

Closes #857

• Tests added/passed
• Fully documented
• Entry in whats-new.rst

cc @drounce

[fmaussion]

One nice side effect of this tool: one can check mass-conservation of the dynamical model at the floating point precision.

Current checks are done for simple beds and for Hintereisferner, and mass-conservation is perfect (yay!). However, we slightly loose mass via the simple calving param (not much but still), but I decided not to care too much as we'll try another calving param soon.

[fmaussion]

Correction: I discoverd a small bug in "calving", this is now also 100% mass-conservative