Change cwd within BMI functions rather than expecting client to do so

[Hofer-Julian]

Current situation
Most BMI functions of MODFLOW6 expect to have mfsim.nam in the current working directory as documented here:

modflow6/srcbmi/mf6bmi.f90

Line 57 in a38723b

!! It is required to have the MODFLOW 6 configuration file 'mfsim.nam'

We take care of that in xmipy so the user typically doesn't have to worry about this as can be seen here: https://github.com/Deltares/xmipy/blob/96976220712f4118629f9ccaf5e4b669cd8d6128/xmipy/xmiwrapper.py#L126
However that also means that wrapper for each language has to do it again.

Proposal
Change the current working directory in the MODFLOW 6 BMI functions.

Regressions
If we then stop changing directory from xmipy, people with old xmipy versions and new MODFLOW6 versions will run into trouble.
Should be easy to avoid by just waiting a bit until we remove that from xmipy.