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 #122, #123, #124, #125 and #126 has been combined into a single file.

• Add unit tests
• Make the record functionality work
• Better documentation
• 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