ENH: Config syntax for importing objects

[rrjbca]

Description

This is a proposal for an extension to the config syntax that allows objects to be imported directly from external (sub)modules using the ! tag and providing neither a list of args or a dict of kwargs. (N.B. to call a function without any arguments pass and empty list of args or an empty dict of kwargs). For example this enables:

cosmo1: !astropy.cosmology.Planck18 # import the Planck18 object and bind to cosmo1
cosmo2: !astropy.cosmology.default_cosmology.get [] # call default_cosmology.get() and bind to cosmo2

Note that as a result of the design, it is no longer possible to call a function with a single argument using a scalar node; the argument must be put into a list:

old_syntax: !len 'hello world!' # invalid
new_syntax: !len ['hello world!'] # correctly define an explicit list of args

Checklist

• [x] Follow the Contributor Guidelines
• [x] Write unit tests
• [ ] Write documentation strings
• [x] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request

[rrjbca]

The syntax will be documented in #454