Lowest order integration rules for tets

[kinnala]

Right now the lowest order integration rule for tets has 4 points. The following rule is suitable for P1 Laplacian though:

quadrature=(np.array([[.25],[.25],[.25]]), np.array([1/6]))

It should be added to get_quadrature.

[adtzlr]

Great, this saves some computation time and memory for linear elasticity.

The only downside (if it is really considered to be one) of it is that projections do not work with this lowest-order quadrature. But it is so obvious that this should not be a problem. I think a typical scikit-fem user has enough knowledge.

[kinnala]

I think it won't be a problem since usually it would not enabled by default, user has to write Basis(..., intorder=1). Perhaps it will be enabled by default if initializing a basis for Basis(..., ElementTetP0())). I will need to check this.

[kinnala]

So basically, before this change, if user initializes Basis(mesh, ElementTetP0()) it chose automatically the rule with 4 integration points while, after the change, it would automatically choose the rule with 1 integration point. So there might be some surprises involved if a user relied on this kind of behaviour.

[kinnala]

I am considering of circumventing the aforementioned issue by increasing ElementTetP0.maxdeg from 0 to 1.

[kinnala]

Nah, I will let the speed benefit show there.