Using undefined parameter in a symbolic rate expression (transitions) slips thru input checks and results in uninformative error

[twallema]

Describe the bug

Very easily made mistake in large configs: I used a parameter I hadn't defined in a symbolic rate expression in the transition block of the config.

Uninformative error stack: doesn't point to real cause of mistake, makes it seem as if there is a mistake in the way the symbolic transitions are parsed. Made me worry Flepi couldn't parse symbolic "divide /" or underscores in parameter names, etc.

parse_parameters (compartments.py)
--> parse_parameter_strings_to_numpy_arrays_v2
--> scimpy.lambdify()
-->scimpy._imp_namespace()
--> ERROR

To Reproduce

seir:
  integration:
    method: rk4
    dt: 1.0

  parameters:
    gamma:
      value: 0.2

  transitions:
    - source: ["S"]
      destination: ["I"]
      rate: ["beta"]
      proportional_to: [[["S"],["I"]]]
      proportion_exponent: ["1","1"]
    - source: ["I"]
      destination: ["R"]
      rate: ["gamma"]
      proportional_to: ["I"]
      proportional_exponent: ["1"]

Very easy mistake to make.

Expected behavior

This not slipping thru the cracks by adding an appropriate input check.