Remove unnecessary allocatable arrays

[mnlevy1981]

We cleaned up KPP before the beta release, but other modules still have allocatable arrays that should have a declared size...

[mnlevy1981]

Did this with one commit per module:

• cvmix_background: cddf7e2
• cvmix_ddiff: ab881a8
• cvmix_shear: 0aef5e7
• cvmix_tidal: 6eee6d1
• cvmix_convection: 97decde
• cvmix_math: 5204d10

There is still an allocatable matrix in the parameters of cvmix_background, which I think is unavoidable. For cvmix_math, I forced a matrix to be 4x4 even though it only needs to be 3x3 in some instances (and isn't used at all in others): it's a matrix in the interpolation routine, so it's only needed for quadratic (as a 3x3) or cubic (as a 4x4) interpolation.