Implement time-heterogeneous rates for CompartmentTypes

[ArtPoon]

For example, the branching rate of a CompartmentType may switch at a given time point for the entire population.
Currently we specify branching rate as

CompartmentTypes:
  host:
      branching.rate: 1.0

We may alter this to:

CompartmentTypes:
  host:
      branching.rate:
          - 0: 1.0
          - 2.5: 0.5

so this rate is 1.0 at the start of forward-time simulation, and then switches from 1.0 to 0.5 at t=2.5.

[RouxCil]

Basic functionality has been implemented. However, error checks need to be implemented and the documentation updated.

[RouxCil]

Documentation updated and basic error checks implemented.

[ArtPoon]

Let's work on merging this next week

[ArtPoon]

Need unit tests for epochs @RouxCil
See also PoonLab/latent-beast#1

[ArtPoon]

In refactored code, it should be possible to add something like sin(t) to the R expression for a rate parameter. We would need to link t to the cur.time variable in simDynamics.R, however.