feature: support writing MF6 binary output files

[wpbonelli]

Currently the binary file handle utilities are read-only. Support for writing MF6 binary output files may be useful in some situations, in particular converting outputs from non-MF6 flow models to MF6 format, so MF6 transport or tracking models can consume them.

[christianlangevin]

Once this is in, we could remove the binary_util.py tools at: https://github.com/MODFLOW-ORG/modflow6/blob/develop/autotest/binary_util.py