added check to length of input args for IndependantOperator

[shimwell]

Description

I noticed it is easy to pass incorrect length arguments to Independent Operator and found myself passing in flux groups instead of single flux values.

After reading the docs string more carefully I now realise that the length of fluxes, micros and materials should all be the same.

Passing in a longer flux with extra values does not actually cause an error in the code, it just continues without a crash.

So it is a bit too easy to add in a flux spectra here without noticing. I added in a check that will catch the input error and a test to test the error handling.

Many thanks to @jbae11

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
• I have followed the style guidelines for Python source files (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[shimwell]

I'm suspicious of this line that gets the length of flux but not energy and doesn't check if they are the same

[jbae11]

nice! Thanks for this @shimwell

[paulromano]

Another one good to go. Thanks!