BSplines of order 0,1,2,3 are support, not 4

After further testing with 4-th order BSplines are not supported. So
they are removed in favor of the 0th order which are actually
implemented.

Change-Id: Ia74b865186bc918b7ffaf841e4a0e6b170563b61
Use unsigned int for bspline order type

Despite only a limited range of values are valid, the unsigned int
type is preferred over the char type as is translates into better
wrapping into a more natural number.

Change-Id: Ibc3fed8da3fbd30c146e8310c17ae67f20b0fb71
Add support for different bspline order in the initializer.

Change-Id: Ia7dff71634668265825b197ed0505e308a1d50c8
Merge topic 'SIMPLEITK-573_AddAdditionalTransformMethods' into next

5e9d334d Add support for different bspline order in the initializer.
16359c46 Use unsigned int for bspline order type
a53d437a BSplines of order 0,1,2,3 are support, not 4