Suggestions for documentation

This is a great piece of software and super helpful for configuring MODFLOW models! I have a couple small suggestions for things I wasn't sure about in the configuration.

1. Most of the examples use TIF files for things like layer elevations, etc. Can we also use shapefiles? If so, is there a way to specify which attribute should be used in a shapefile? (For example, we have a shapefile with a model grid made by an initial cut at the model with `mfsetup` and sampled elevations to it but it has multiple attributes).
2. For time series like recharge, the examples mostly use netCDF which is great but not always available. You explained to me offline that one can provide individual files per period which obviates the need for `resample_method` and `period_data` information. I didn't find the specific example YML file that used this approach as I browsed, however, so would be good to have an explicit description
3. The nature of the `source_data` block could be explained a bit more explicitly. As I was hacking examples, I tried replacing files used for `botm` for example with single values in the `source_data` not understanding that `source_data` was only for files.