compute does not appropriately handle callback #311

Using a fix provided by Maarten, have compute properly adjust the
callback messages to account for tiling.
shapefile iterator may also yield spatial dimension objects #305

Spatial dimension objects are now used throughout the code including
in the subset module. Many of the external functionalities (i.e.
rotated pole management, wrapping/unwrapping) are now attached to
the spatial dimension. The subset module has undergone a considerable
refactor.
Merge branch 'next' into i309-moving-window

Conflicts:
	src/ocgis/test/base.py
	src/ocgis/test/test_ocgis/test_interface/test_base/test_dimension/test_spatial.py
mark - kernel iterator working
mark - added check for mode
mark - moving average tests working
mark - all tests working