Purge/deprecate outdated element terminology

Major overhaul to replace outdated element terminology in the grid and
affiliated framework. Mainly achieved by deprecating existing
functions, so everyone’s code should still work.
Key changes:
* INTERIOR_NODE => CORE_NODE
* INACTIVE_BOUNDARY => CLOSED_BOUNDARY
* Most references to active_nodes & active_cells have been deprecated,
or at very least docstrings clearly record that this refers to core +
open_boundary nodes/cells.
* Added numerous functions referring to core to supersede deprecated
methods.
* Updated as many docstrings as I could, probably lots of duff
terminology got past me. Please correct it when you find it.
Merge branch 'master' of https://github.com/landlab/landlab

Conflicts:
	landlab/grid/base.py
	landlab/grid/raster.py