Generalize 1D and 2D data

[krzywon]

As noted in sasview/sasview#841, the 1D and 2D data objects could be abstracted out into a single class. This would allow more generic data formats to be added as needed.

In this package, the following is required:

• [ ] Combine the plottable_1d and plottable_2d classes into a single class (called plottable?)
• [ ] Combine the Data1D and Data2D classes into a single class (called Data?)
• [ ] Update all base file loader class
• [ ] Update all specific reader classes
• [ ] Update file conversion routines
• [ ] Update manipulations (currently only works for 2D data and there are checks for this)
• [ ] Update unit tests to match new functionality

[lucas-wilkins]

Solutions to this might be relevant to https://github.com/SasView/sasview/issues/506