Load data without unit conversion and store base units

[krzywon]

As noted in SasView/sasview#1465, as data is loaded, the values should not be converted to other units for many reasons. In this package, the as-loaded units should held in a separate class variable than the current units so data can be scaled by the user as desired.

[butlerpd]

Working on cleanup in preparation for Contributor Camp XII:

@krzywon Is this part of the #28 PR on generalizing the data object? If so can we link it? if not I guess this is not a "good first issue" either 😄 ?

[krzywon]

Is this part of the #28 PR on generalizing the data object? I guess this is not a "good first issue" either 😄 ?

Correct on both. I'll link the issue once the work is added to the PR.