Document model_params and add examples to output_header_fields and output_variables

[mattw-nws]

Document model_params and add examples to output_header_fields and output_variables in https://github.com/NOAA-OWP/ngen/blob/master/doc/BMI_MODELS.md

Current behavior

Items 2 and 3 above are documented but an example would be helpful. No examples found in code. Item 1 is not documented and needs to be added (see PR #368 and Bmi_C_Formulation_Test.cpp line 210).

Expected behavior

An example should be provided for output_variables and output_header_fields at least similar to the one given for variables_names_map. Preferably, adding these to some multi-bmi realization file as an example would be helpful if it doesn't break any tests (it probably wouldn't). The scenario to be demonstrated is how to obtain multiple output variables from a combination of modules in a multi-BMI formulation.

Also, documentation should stipulate that output_variables and output_header_fields should (must?) be specified at the top level of a formulation rather than the module-level configs in a multi-BMI config.

Similarly, model_params must be specified on the module config instead of the top-level formulation config, and this should be specified.

Consider adding an applicability table similar to the one under the Required Parameters section for the optional parameters--though this would specify where they were allowed rather than where they were required.

Steps to replicate behavior (include URLs)

Screenshots

[mattw-nws]

Adding mention of #588 to this, since the undocumented thing has changed since this issue was created.