Feature: allow different snap modes used for grid size and offset

[mwtoews]

This feature adds more control on the evaluation of grid size and offset from input bounds, which is used with most Grid classmethods.

The default snap mode full will snap bounds to a multiple of resolution, which has been the only mode up until now.

Some other modes include:

• half: snap to half-resolution, for instance when creating a finite-element grid where the representative value is at grid corners, thus it need to be shifted by a half grid resolution.
• top-left, top-right, bottom-left, or bottom-right: snap to corner, without rounding any digits.
• A coordinate tuple (snapx, snapy): directly specify a point on the grid, although the grid may not necessarily include the coordinate if it is outside.

There are a few instances where the behaviour has changed, only when a buffer distance was also provided. These previous grids were not large enough to contain the original bounds, and is corrected here.

Also, some classmethods have been modified to force some keywords to be specified (rather than just positional).