Arbitrary constraints on GL geometry

In gl_space.py on current master, there are constraints on the grid layout which in my opinion make no sense.

• in _parse_nlat, the number of rings is not allowed to be odd. Why ist this the case? I would argue that any positive number of rings is allowed and useful.
• in _parse_nlon, a warning is given if nlon!=2*nlat-1. Depending on the actual values of lmax and mmax for the codomain, this choice is not necessarily the best one.

For GL grids, the connections between nlat, nlon, mmax and lmax should be the following:

• nlat = lmax+1 (this is always a good choice, and a warning could safely be given if the user specifies something different)
• nlon >= 2*mmax+1 (this is the minimum number of pixels per ring that can represent spherical harmonics up to and including mmax. However, it may be preferrable to choose a slightly higher nlon for performance reasons: if possible, nlon should be a product of prime factors <=5).

I am aware that the constructor for GLSpace currently does not have the desired lmax and mmax values at hand, and so cannot warn if the relations above do not hold. However, the current tests should be changed, since they are arbitrary and their restrictions do not improve the code in any way.