Add ElementHex2

[kinnala]

Here is some work-in-progress how ElementHex2 could look like (not fully working yet I'm afraid). There are some optimizations possible, but I need to measure it first. Closes #327 when ready.

[kinnala]

Something's wrong, most likely ordering of the DOF's.

[kinnala]

I'm slightly confused that none of the unit tests catch this issue and only the convergence test fails.

[kinnala]

Yeah, had doflocs in the wrong order. 🤦

[kinnala]

I'm now wondering if this way of constructing the basis should be a general strategy: start with ElementGlobal on the reference element and then transform that into ElementH1.

[kinnala]

I'll leave it like this for now; now that the tests exist and the implementation is correct, we can optimize it later if needed.