Allow user to set model parameters for simulating trees

[ArtPoon]

@MathiasRenaud noted a use case where a user may want to simulate trees under specific parameter settings of a model. We discussed adding an "initial parameter values" setting to the Kaphi configuration object, but instead I think the appropriate method would be to specify Dirac delta functions as the prior distributions for the respective parameters.

Actions:

• [x] Need to look into whether delta functions are available in R.
• [x] Incorporate this method into writing unit tests.

[MathiasRenaud]

The current method to be used as an alternative to delta functions for setting specific parameters is to set the priors in the config file as a uniform distribution with hyperparameters 'max' and 'min' both set to the desired value.

priors:
  lambda0:
    dist: 'unif'
    hyperparameters:
    - min: 0.1
    - max: 0.1
  lambda1:
    dist: 'unif'
    hyperparameters:
    - min: 0.2
    - max: 0.2
  mu0:
    dist: 'unif'
    hyperparameters:
    - min: 0.03
    - max: 0.03
  mu1:
    dist: 'unif'
    hyperparameters:
    - min: 0.03
    - max: 0.03
  q01:
    dist: 'unif'
    hyperparameters:
    - min: 0.01
    - max: 0.01
  q10:
    dist: 'unif'
    hyperparameters:
    - min: 0.01
    - max: 0.01