Issue #344 part1 imod refactor primod update

[JoerivanEngelen]

Closes #344, this is the first task to update primod for the iMOD Python refactor.

Plans are layed out in #342. In this first task, I changed the use of the deprecated WellDisStructured object, to the Mf6Wel object, which is a direct translation of MODFLOW 6's cellids to a file. As this iMOD Python refactor is still work in progress, I temporarily introduce a install-imod task, which pip installs imod from a git branch, instead of a release on conda-forge. After I have accomplished part 2 of the refactors, I'll make an iMOD Python release, and this can be reverted back.

It does the following things:

• Temporarily introduce a install-imod task, which install imod from a git branch
• Use the Mf6Wel object instead of the WellDisStructured object
• I had to drop an unused layer coordinate in a RibaMetaMod fixture
• I set the fill in calls to imod.prepare.rasterize to -1, to work around this issue Primod's basin mapping can't deal with integer basin id's with latest Rasterio dependecy  #333 (tests seem to succeed?)