Implemeted MRI IFU empirical cruciform

[patapisp]

This is the the start of a PR for finalising the broadening and cruciform implementation for the MIRI IFU mode. Added an additional option for model_type = "cruciform"

Remaining tasks:

• check normalisation convention when modifying PSF models
• changing the oversample parameter yields quite significant differences in the PSF models
• switch empirical broadening (now following Law+2023) to broadening due to sampling by detector and cube pixels

[pep8speaks]

Hello @patapisp, Thank you for submitting the Pull Request !

• In the file webbpsf/detectors.py:

Line 609:64: E226 missing whitespace around arithmetic operator
Line 610:102: E226 missing whitespace around arithmetic operator
Line 611:106: E226 missing whitespace around arithmetic operator
Line 698:23: E226 missing whitespace around arithmetic operator
Line 741:21: E226 missing whitespace around arithmetic operator
Line 741:25: E226 missing whitespace around arithmetic operator

If you have not done so, please lint your code in accordance with best practices.

[mperrin]

I'm noting here for the record that the workflow on this one is going to be atypical. Rather than giving a code review to Polychronis and then having him revise the PR, I'm going to take over and be editing and finishing this one myself.

[mperrin]

See #930 instead