Grid partitioner silently ignores and overrides invalid user parameters

[GiudGiud]

Bug Description

If the user specifies nx, ny and nz such that the product does not amount to n_MPI, these three user-specified values are ignored and instead we use the MPI routine to get a Cartesian grid.

This is not ok. We don't ignore user parameters ever, especially without telling them.

Steps to Reproduce

use 12 processes and specify nx = 4 ny = 4 nz = 1

Impact

Waste of time when running scalability studies and getting jumps in the timings due to a different (more optimal most likely) selection of the partitions

[Optional] Diagnostics

No response