standardize approach for defining calibration crosses

[sophiamaedler]

For the selection process calibration crosses need to be defined that can be used for shape alignment between imaging coordinate system and slide coordinate system on the LMD microscope.

It would be desirable to have a simple workflow how this list of points can be stored in the spatialdata object for access during the selection process.

Requirements:

• there should be a possibility to define the points interactively in the Napari-viewer but also to upload them from a csv file and/or list
• we should be able to store multiple calibration point sets
• during selection we should still be able to overwrite the automatic use of points with custom points if desired

[sophiamaedler]

it is now possible to save layers using the interactive Napari viewer by executing SHIFT + E (see #189). This can be used to define new calibration crosses that should be saved to the sdata object.