Add MINI element, and convergence test for RT0

[kinnala]

Solves the same Poisson equation as the other convergence tests.

[kinnala]

Added also MINI element for demonstrating simple Stokes elements. Didn't test with Stokes, but added convergence test for standard H1 case. Could also do some sort of automatized stability testing for the Stokes operator but it's maybe more related to #241 .

[gdmcbain]

Looks good. I'll be interested to try or see these elements in some full examples.

[gdmcbain]

This bilinear form arises in a few places; it might be a candidate for skfem.models.

scikit-fem/docs/examples/ex03.py

Lines 20 to 22 in 46a75bb

	@bilinear_form
	def mass(u, du, v, dv, w):
	return u[0] * v[0] + u[1] * v[1]

scikit-fem/docs/examples/ex21.py

Lines 19 to 21 in 46a75bb

	@bilinear_form
	def mass(u, du, v, dv, w):
	return rho * sum(u * v)

https://github.com/gdmcbain/fenics-tuto-in-skfem/blob/119258655559ab7ac6b563bf75be6923d494d2be/07_navier_stokes_channel/st07_navier_stokes_channel.py#L12-L14