Swap slow fast to fast slow when setting beam centre

[dagewa]

It is a source of occasional annoyance that setting the beam centre by pixel coordinates in DIALS requires passing it in the order slow, fast [panel], whereas the user may expect fast, slow, [panel] as a more natural ordering. Indeed, in a new tutorial it was noted that dials.generate_mask untrusted.circle=1059,988,100 accepts values in the order fast, slow, radius.

This PR introduces fast_slow_beam_centre as a new way to set the beam centre. The previous parameter, slow_fast_beam_centre continues to exist, but will be overridden by the fast, slow version if both are set.

A parallel PR in the DIALS repository will change the display in the image viewer to the fast, slow order.

[codecov]

Codecov Report

Merging #421 (1d8ccf1) into main (23c3a9b) will increase coverage by 2.17%.
The diff coverage is 90.37%.

❗ Current head 1d8ccf1 differs from pull request most recent head 3014968. Consider uploading reports for the commit 3014968 to get more accurate results

@@            Coverage Diff             @@
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+ Coverage   64.46%   66.64%   +2.17%     
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  Files         177      183       +6     
  Lines       15309    16580    +1271     
  Branches     2077     2181     +104     
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+ Hits         9869    11049    +1180     
- Misses       4896     4973      +77     
- Partials      544      558      +14     

[ndevenish]

but will be overridden by the fast, slow version if both are set

Can we make this an error instead if both are specified (or specified differently)

[dagewa]

Thanks for the suggestions, I think I have dealt with those now. dials/dials#1849 is intended to be merged at the same time.