Add add_observable function with a concept pattern

[bgyori]

Per a suggestion from @liunelson, we should implement a function along the lines of

def add_observable(template_model, name, concept_pattern):
    # Find all concepts that match or are refinements of the concept pattern
    # Create an Observable which is a sum of all the above Concepts with the name provided as the input parameter
    # Add the Observable to the template model

This could be implemented as a standalone function or a method of the TemplateModel class.

[liunelson]

A use case from which my suggestion originated is as follows:

• I start with a basic SIR compartmental model
• I stratify the state vars S, I, beta by vaccination status u, v
• The model now has states S_u, S_v, I_u, I_v, R and parameters beta_u_u, beta_u_v, beta_v_u, beta_v_v, gamma
• I stratify it again, on the state vars S_*, I_* by age group y, o and location us, eu
• The new state vars are X_i_j_k where X = S, I, i = u, v, j = y, o, k = us, eu
• I'm asked to do some model configuration checks on the sums of the state vars; for example, the total unvaccinated population should never increase; however, it is very tedious to type in every expression and add them as new observables.
• These new observables could be:
  N = \sum_{X, i, j, k} X_i_j_k (total population)
  N_XX = \sum_{X, i, j, k} X_i_j_k * kroneckerdelta_{X, XX} (e.g. N_S total susceptible population)
  N_ii = \sum_{X, i, j, k} X_i_j_k * kroneckerdelta_{i, ii} (e.g. N_u total unvaccinated population)
  N_jj = \sum_{X, i, j, k} X_i_j_k * kroneckerdelta_{j, jj} (e.g. N_o total old population)
  N_XX_ii = \sum_{X, i, j, k} X_i_j_k * kroneckerdelta_{X, XX} kroneckerdelta_{i, ii}(e.g.N_S_u total unvaccinated susceptible population) ... repeating for each stratum group (u, v, y, o, ...`) and stratum (disease status, vacc status, age group, location) combination...

[bgyori]

In #364 I implemented the ability to very easily add observables by pattern e.g., "total old population" or "total susceptible population" irrespective of how many distinct compartments these are divided into.