Add algorithms categories that are by default hidden in the algorithm listing

[mantid-builder]

Original Reporter: Russell Taylor

This ticket is blocks : TRAC3402

There are some classes of algorithms that should in general not be used by the typical Mantid user. Examples include:

• Algorithms under development but not yet finished
• Algorithms that are only really there to aid developers (e.g. Pause, NexusTester)

One solution would be to put these in separate libraries that are not shipped by the installer; a simpler one is to have hidden categories and what needs to be done is the following:

• Come up with appropriate category names.
• These names will be 'special' in the sense that they should be hidden by default in the algorithm tree - the appropriate changes should be made to MantidPlot to allow this. The algorithms should still be available if typed manually - e.g. in the text box or in Python.
• Algorithms should be moved into these categories where appropriate.
• Developers should be notified of the availability of these categories and should start to use the 'in developement' one for new algorithms that are likely to take a while to fully develop (and then change the category on completion).

[mantid-builder]

This issue was originally trac ticket 5209