Initial (type / dimension) checks for packages

As a follow-up of issues #148 (closed) and #146 (closed), add a fast initialization check to packages, in addition to the more thorough value / nodata checking that is currently implemented (and described in #74). The latter requires other packages to be present, loading of values in memory and is usually deferred to just before rendering the model.

The fast initialization check includes:

• type checking: floats if floats required, ints if ints required
• proper dimensions in resulting package (layer, y, x)