Add `md.cluster.marshalldir` to specify where binary files should be marshalled

[lawrenceabird]

When working with ARE on Gadi, $HOME is set to your default home directory. By default, pyISSM will marshall binary files to $HOME. Due to storage limitations in Gadi users' home directories, this can quickly become problematic when working with large binaries.

Suggest adding md.cluster.marshalldir to allow users to specify where binary files should be marshalled. This can default to $HOME (as is current behaviour), but allows things to be updated. This should be fairly easy to implement and may only touch pyissm.model.execute.marshall, pyissm.model.classes.toolkits and maybe pyissm.model.execute.solve.

[lawrenceabird]

For now, simply specifying os.chdir('/path/to/marshall/directory') overcomes the issues of Gadi ARE defaulting to $HOME. This allows large bin files to be saved to disk in any specified location.

[MathieuMorlighem]

FYI in ISSM files are now marshaled in your execution directory by default. Would that help?