Allow typed dictionaries, and dictionary subscripts from expander

[douglasjacobsen]

This merge enables the expander to handle dictionaries better.

The most obvious change from this is that given:

variables:
  test_dict:
    key1: val1
    key2: val2

then '{test_dict["key1"]}' should expand to "val1".

A more subtle change from this is that given the variable definition above, inside object definitions one can now do:

test_dict = app_inst.expander.expand_var_name("test_dict", typed=True)

for key, val in test_dict.items():
  print(f"Key {key} = {val}")

Tests and documentation have also been added for this.