Add a method to show users all valid measurement types.

[CFlaniganMide]

Add a helper function (probably as a class method) to MeasurementType which would generate and ascii table of the measurement types for users to reference.

[StokesMIDE]

Something like that would be good. Maybe a function instead, that would also include the 'constant' variable names in the module - for convenience, some constants reference the same MeasurementType instance.