Newton-Krylov Solver attempts to run with divergence gradient calculation but shouldn't.

[uqngibbo]

Import of issue #9 from the old repository, originally reported by Lachlan Whyborn.

Lachlan Whyborn created an issue 2021-12-04

The Newton-Krylov solver requires the least-squares gradient calculation for the construction of the numerical Jacobian, but will still attempt to run if ‘divergence’ is selected for the spatial_deriv_calc. Should probably throw a more descriptive error during initialisation rather than an unitialised least-squares workspace error.