Infer units for numeric literals

[artgoldberg]

Because numeric literals are 'dimensionless', expressions cannot use them as terms that must have dimensions. E.g, consider this rate law: (k1 * P[c] * (P[c] - 1)) / 2.

Validating the model that contains it generates this error:

ValueError: The model cannot be loaded because it fails to validate:
  Parameter:
    1:
      'id':
        Value '1' does not match pattern: ^(?!(^|\b)(\d+(\.\d*)?(\b|$))|(\.\d+$)|(0[x][0-9a-f]+(\b|$))|([0-9]+e[0-9]+(\b|$)))[a-z0-9_]+$
  RateLaw:
    Dimerization-forward:
      'units':
        Exception: cannot eval expression '(k1 * P[c] * (P[c] - 1)) / 2' in RateLawExpression; Cannot convert from 'molecule' ([substance]) to 'dimensionless' (dimensionless)

I suggest that if the units of a numeric literal can be unambiguously determined, then they should be determined.