Attempt at making substitutions work inside dicts and lists.

[JSKenyon]

This PR aims to improve substitutions by supporting them inside compound types. This probably needs extensive testing, but the following should now work:

inputs:
  foo:  # foo is Dict[str, Tuple[File, float, str]] type.
    bar: [=expression_with_substitution, 1, example]  

or equivalently:

inputs:
  foo:   # foo is Dict[str, Tuple[File, float, str]] type.
    bar: =LIST(substitution, 1, "example")

This should also support more complicated cases e.g.:

inputs:
  foo:   # foo is List[Dict[str, Tuple[File, float, str]]] type.
    bar: 
      - baz: [=expression_with_substitution, 1, example]

[o-smirnov]

@JSKenyon clever, clever. Maybe use (list, ListConfig) just in case?

[o-smirnov]

Even better:

params_out[name] = type(value)([x for x in
    self.evaluate_dict({f"[{i}]": v for i, v in enumerate(value)}, ...).values()
   ])

...because then any nested error messages will report the location of the error sensibly as "[n]".

[JSKenyon]

Ah, awesome. Definitely better than a profusion of dummies!

[o-smirnov]

Actually no need for the extra list comprehension, right? This should work too:

type(value)(self.evaluate_dict({f"[{i}]": v for i, v in enumerate(value)}, ...).values())

[JSKenyon]

Ok, I have made a final change. Let me know if it appeals.

[o-smirnov]

Oh, also the evaluate_dict() call needs sublocation=sublocation + [name], so that errors are reported from the correct nested namespace.

[JSKenyon]

Thanks for making those changes @o-smirnov - was only going to get to it today.