Adding plugins for doing processing of differential phase contrast datasets

[magnunor]

Adding plugins for doing processing of differential phase contrast datasets.

The plugins made in https://github.com/hyperspy/hyperspyUI/pull/122, https://github.com/hyperspy/hyperspyUI/pull/123, https://github.com/hyperspy/hyperspyUI/pull/124, https://github.com/hyperspy/hyperspyUI/pull/125 and https://github.com/hyperspy/hyperspyUI/pull/126 has been combined into a single file.

• [x] Add unit tests
• [ ] Make the record functionality work
• [ ] Better documentation
• [x] Fix comments from last pull requests

[magnunor]

Any good examples on how to make the recording functionality work?

[vidartf]

Looking at the code, I'm not entirely sure how to best record as code: Almost all your entry points start off with select_x_signals which is hard to record well as a macro code. Maybe it is best to simply leave it out, and have the user rely on recorded actions?

[magnunor]

Added unit tests for the processing itself