Support for variable-time models

[AndrewIOM]

Description

Some datasets do not have equal time-steps, such as those derived from sediment core age-depth models. Bristlecone should fully support models for these data.

Implementation details

This requires an integration routine that supports variable time-steps. An example is currently included for a single-equation model system at Bristlecone.Integration.Simple.rungeKutta4Variable'.