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 SasView/sasview#506