Add !list and !dict UW YAML tags

[maddenp-noaa]

Description

Add support for !list and !dict UW YAML tags to convert quoted values, e.g. Jinja2 expressions, into Python list and dict objects. Among other use cases, this may help avoid the use of wildcards in file paths in e.g. fs mode configs, while keeping them DRY.

For example config.yaml

as: !list "[{% for n in range(3) %} a{{ n }},{% endfor %} ]"
bs: !dict "{{ '{' }}{% for n in range(3) %} b{{ n }}: {{ n }},{% endfor %}{{ '}' }}"
cs: !dict "[{% for n in range(3) %} ['c{{ n }}',{{ n }}],{% endfor %} ]"

would currently be rendered as

$ uw template render --input-file config.yaml
as: !list "[ a0, a1, a2, ]"
bs: !dict "{ b0: 0, b1: 1, b2: 2,}"
cs: !dict "[ ['c0',0], ['c1',1], ['c2',2], ]"

Recall that, since config realize renders values in key-value configs on their own, this is a realistic view of a potential intermediate state of the config during dereferencing. After implementation of the new tags, these values would then be converted to primitive Python values like

>>> list(yaml.safe_load("[ a0, a1, a2, ]"))
['a0', 'a1', 'a2']
>>> dict(yaml.safe_load("{ b0: 0, b1: 1, b2: 2,}"))
{'b0': 0, 'b1': 1, 'b2': 2}
>>> dict(yaml.safe_load("[ ['c0',0], ['c1',1], ['c2',2], ]"))
{'c0': 0, 'c1': 1, 'c2': 2}

Note that the final values of bs and cs are equivalent, but the list-of-lists representation in the original Jinja2 expression was more concise, due to not having to escape braces.

AC

• Implement tags.
• Add/update unit tests.
• Update documentation.