as_midp parameter per axis

[kohr-h]

There is an overlooked issue with grids and the handling of as_midp when combining two grids, which is common in tomography when there is a separate grid for the angles and one for the detector. We have this situation:

>>> grid = odl.RegularGrid(0, 1, 10, as_midp=False)
>>> other_grid = odl.RegularGrid(-1, 1, 20, as_midp=True)
>>> grid.insert(other_grid, 0)
>>> grid.as_midp
False

The problem is that as_midp is global, and currently the inserted grid's as_midp is silently ignored. We should make as_midp a property per axis and then set it properly when inserting. This fits well into #159, the original issue where this first came up was #172.

[kohr-h]

This is now handled gracefully in partitions. Closing.