Maintenance for the SANSTubeCalibration algorithm (#39057)

* Improve error msg when getting workspace indices for tube

* Add type annotations to SANSTubeCalibration

* Add type annotations for SANSTubeMerge

* Add docs for FlatTopPeak function

* remove stringized typing in favour of importing annotations from __future__

* add image to FlatTopPeak doc

* add image to commit

correct extension

reduce image size

fix typo

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Co-authored-by: Rachel Baust <rachel.baust@stfc.ac.uk>

Author: MialLewis