fix bug in AmiAverageStep spec that prevents step creation

[braingram]

The step spec for AmiAverageStep contains an invalid entry:

jwst/jwst/ami/ami_average_step.py

Lines 14 to 16 in 7d97170

	spec = """
	skip = True # Do not run this step
	"""

which leads to an error when attempting to construct the step:

ValidationError: Config parameter 'skip': missing

This PR fixes the spec and adds a very basic unit test to verify the step can be instantiated.

Checklist for PR authors (skip items if you don't have permissions or they are not applicable)

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• All comments are resolved
• Make sure the JIRA ticket is resolved properly

[braingram]

Given that this step couldn't be created I am inclined to skip regression tests as the step is effectively untested.

[tapastro]

A question related to this PR: do you know if NIRISS expects this step to be used? It is not part of the reworked AMI pipeline, right?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.47%. Comparing base (7198d89) to head (cf91724).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8677      +/-   ##
==========================================
+ Coverage   60.36%   60.47%   +0.10%     
==========================================
  Files         372      369       -3     
  Lines       38320    38408      +88     
==========================================
+ Hits        23132    23226      +94     
+ Misses      15188    15182       -6     

☔ View full report in Codecov by Sentry.
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[braingram]

A question related to this PR: do you know if NIRISS expects this step to be used? It is not part of the reworked AMI pipeline, right?

Thanks for giving it a look. I don't believe they expect this to be used (currently) based on the comment in the description for #7862:

ami_average:

This step will be turned off in the default AMI3 pipeline flow until the effects of averaging together multiple reference star observations are better understood. The code is left as-is except "skip = True" has been added to the spec block. This step will not work if run manually on the AmiOIModel products.

Perhaps @rcooper295 knows more.

I only noticed the issue fixed in this PR because I tried to create a basic instance of every step (to try and wrangle the suffix calculations since that appears to be one limitation for removing the deprecated cfg files in the pipeline directory. See #8664 for not really details but at least the current failures with the cfg files removed).

[tapastro]

Pending rebase, LGTM