Implement the support for FLOOD file for FRONT detectors

 - SANSRunWindow.ui: add two file widgets to pass the FLOOD FILE.

 - SANSRunWindow.cpp: adapt this class to deal with two FLOOD FILE.

 - isis_reduction_steps.py:
   - Override the setPixelCorrFile of CalculateNorm in order to accept flood files for FRONT and REAR detectors.
   - Work around the calculate method, in order to passe to CalculareNorm the correct _pixel_file to execute the pixel dependend correction.
   - Implement the parser for the  user file to accept the new commands: MON/FLAT/[REAR/FRONT].

 - ISISCommandInterface.py:
   - Adapt the SetDetectorFloodFile function to the new setPixelCorrFile.
   - Remove the [https://github.com/mantidproject/mantid/commit/7ceaa78b4ad5d7f25cd44351e4e52e75429a1341/ changeset] that put to ignore the FLOOD file for Front detectors.

 - ISISCommandInterface.py: Adapt the SetDetectorFloodFile to support the new setPixelCorrFile method.

 - isis_instrument.py:
   - Add two methods that provide a way to access the detectors bank independently from their names (which is confusin, because SANS2D uses different names from LOQ)
   - Solve a problem with crop_to_workspace that was causing the HAB FLOOD file to break because it was trying to crop mixing the definition of workspace index and spectrum indexes.

re #6061
Remove useless warning that is not valid any more

Now we support FLOOD file for HAB.

re #6061
Bring back the user file parsing

Some system tests breaks with the previous one.

re #6061
Correct issues regarding the FindCentre algorithm

re #6061
Merge branch '6061_flood'