Added reader base class and reworked nikon reader to use it

[gfardell]

Describe your changes

• Added a reader base class with new functionality

  • metadata and geometry as properties of the object
  • get_raw_flatfield - returns the flatfields as a numpy ndarray (no processing/casting/cropping)
  • get_raw_darkfield - returns the darkfields as a numpy ndarray (no processing/casting/cropping)
  • get_raw_data - returns the data as a numpy ndarray (no processing/casting/cropping)
  • set_panel_roi - set the ROI in the horizontal and vertical directions, allows cropping and binning
  • set_projections - set the projections to read in by index as a slice or list
  • get_geometry - returns a CIL AcquisitionGeometry describing the data respecting ROI and angle selection
  • read - returns a CIL AcquisitionData the normalised data respecting ROI and angle selection

• Methods marked as abstract that must be defined by a child before the class can be instantiated - Is this what we want?

• Updated the NikonDataReader to use the base class

Describe any testing you have performed

Please add any demo scripts to CIL-Demos/misc/

Link relevant issues

closes #1020
closes #1144

Checklist when you are ready to request a review

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• I have implemented unit tests that cover any new or modified functionality
• CHANGELOG.md has been updated with any functionality change
• Request review from all relevant developers
• Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License.
• I confirm that the contribution does not violate any intellectual property rights of third parties