Parseq

- Enable custom node icons.

- Enable node arrays with an extra abscissa.

- Add glitch detection.

- Add metavariables to fits.

- Enable several transformation widgets per node, with the idea to get a
widget in the 'corrections' splitter.

- Add optional interpolation when making data combinations.

- Add Principal Component Analysis and Target Transformation to data
combinations.

- Minor new features and minor bug fixes.

bug fix, a missing file discovered

- Finalize general data corrections. Include doc pages with descriptions and animated examples. Add "delete spikes" data correction.

- Minor new features

- Add general data corrections.

- Add EXAFS fit. It features two fit intervals (k and r), constraints,
ties to other parameters (also of another data item) and error analysis.

- Update the main help pages.

Curve fitting facilities. Implemented 2 general fitting routines: linear combination fit and function fit. Both fits can be tested with `tests/test_fit.py`. Both fits added to parseq_XAS. Added chapters to docs about fits.

Minor new features
Minor bug fixes after tests in many OSes