This also implements direct manipulation of OpenMP settings (schedule
and number of threads) via python.

Now, if either vertex or edge filtering is enabled, simultaneous
filtering is activated. By not considering vertex or edge filtering in
completely independent fashion, the compile time memory requirements
reduced to about 50%.

This fixes ticket #167

This fixes problems with transposed arrays, or arrays with arbitrary strides.

Now, as usual, the position values are returned, instead of the inline
image object.