Create a base unit class

[mantid-builder]

Original Reporter: Russell Taylor

This ticket is blocks : TRAC2507

We have a few cases where we have essentially (or literally) a string representing a unit. However, our Unit class only deals with units that can be converted to and from TOF. We should create a Unit base class which can be used by anything that needs to represent any kind of unit, and then turn the current Unit class into a ConvertibleUnit (or better name) that inherits from the new Unit base class.

An extension of this could be thinking about conversions for this - e.g. from seconds to microseconds.

[mantid-builder]

This issue was originally trac ticket 1380