Implement units within our workspaces

[NickDraper]

After consideration this should not use the gnu units program, this would be over complicated and perhaps not support what we need anyway.

LibIsis takes a simpler approach where it supports a limited number of units
[http://web2.isis.rl.ac.uk/libisiswiki/units%20code].

We need to understand how LibISIS performs it’s units onversion, and implement sonething similar as an algorithm (or collection of algorithms). Talk to Dickon to get an idea of how LibISIS does it.

We would need to store a unit for the Y and X values.
We will also need to store a bool for each to indicate if the data is a distribution.

The units string should be as standard as possible use the settings in UDUnits (google it) as examples(i.e. no pluralisation of the unit). However do not include the use of UDUnits itself.

After speaking to Freddie it appears that for the conversions we can probably get away with using a common unit as a middle man in all conversions. Eg. to convert from wavelength to energy you first convert wavelength to TOF and the TOF to energy. this reduces the number of conversions you need from n*n to n+n.

[NickDraper]

This issue was originally trac ticket 94