Restructure and cleanup BackendMantidReducer

[SimonHeybrock]

The reducer keeps growing, and has gotten messy. We need to restructure it. We need the following:

• run a (fixed) reduction script (flexible may come later)
• rebin result of reduction
• keep event data
• filter event data (keep in mind that FilterEvents can result in multiple output workspaces!)
• rebin filtered data
• add and remove filters
• keep several sets of data (unfiltered, filteredA, filteredB, ...)

[SimonHeybrock]

Commits 92d182b to 7f2ad12 are the major part of a restructuring process to solve this. This should now be in a state where we can think about adding new functionality, after some more smaller cleanup steps.

[SimonHeybrock]

This is mostly done. There still are some minor issues, and I opened #35, which carries on parts of the content of this ticket.