Ignore masked detectors in smooting

[OwenArnold]

Nearest neighbours will ignore monitors, but it also needs to ignore masked detectors.

This can be acheived by.

• Ensuring that the masking algorithm clears the nearest neighbours map on the workspace.
• Have the nearest neighbours map ignore masked detectors when it builds the map.

Note that when using the radius method of smooth neighbours. Users will end up with less neighbours than they used to have when this is implemented (if their are masked detectors). Using the 'Exact' method of selecting neighbours they will end up with the same number since the NearestNeighbours should keep looking for neighbours until it meets the specified value (while still ignoring masked ones). Pascal also wants the option to override this if possible, so that he can select 8 nearest neighbours, but say only end up using 7 if one is masked.

[OwenArnold]

This issue was originally trac ticket 4150