Implement dm_plex_metric_restrict_anisotropy_first

[jwallwork23]

          This should be easy enough to implement (just tweak the arguments to the `enforce_spd` call in the definition of `normalise`). However, I'd prefer to address it in a separate issue rather than here, considering the impact with some tests.

Originally posted by @jwallwork23 in #99 (comment)

Note that the parameter already exists on the PETSc side but is unused by Animate because it implements normalisation itself rather than with PETSc.