Improve grid API

[benbovy]

Make grid API more consistent (some breaking changes).

Closes #129.

• [x] Remove the _boundary suffix of node_status values
  • this is more succinct and consistent with the fact that inner grid nodes may have arbitrary status
  • this has also be renamed in landlab 2
• [x] More consistent grid method names that follow more strict conventions
  • nodes_xxx(): access to node-specific grid information
  • neighbors_xxx(): access to neighbor-specific grid information
• [x] Expose two overloads for nodes_status
  • nodes_status(): get status of all grid nodes as an array
    • read-only -> const (C++) or return a copy (Python)
  • nodes_status(idx): get status at a given grid node
• [x] Expose similar overloads for nodes_areas
• [x] Expose nodes_indices() to Python
  • [x] fix bug no selected node (add C++ tests)
  • [x] update user guide
  • [x] python tests (raster, mesh)