Add an mpas_common and mpas-example script

These scripts are the initial attempt at making the Python script more
declarative than normal scripts. Completely untested.
Add support for PassArrays by default

The functionality is common enough that having it built-in is worth the
extra 3 lines if configuration is much less.
Remove complex parts of the documentation

They're not needed most of the time, so cluttering the docs with them is
wasteful. If they're needed again, they're in git.
Support module chaining
Rename and document the example dataset entry
Rename 'layer' to 'grid'
Remove potentially confusing entries
Rename 'in_situ' to 'web_view'
Improve documentation in mpas-example