Dynamic-beast

Added support for model-selection.

Path sampling options for the package [model-selection](https://github.com/BEAST2-Dev/model-selection) can be add by using the `--gss` option. This will add the default model-selection options (e.g. stepping stone) which can then be configured at runtime with `-D`.

bash
Create dynamic MC3 XML
dynamic-beast --gss --outfile dynamic_gss_hcv_coal.xml hcv_coal.xml
Configure MC3 with BEAST
beast -D "gss.doNotRun=true,gss.rootdir=$(pwd)" dynamic_gss_hcv_coal.xml

Added support for CoupledMCMC.

MC3 options for the BEAST package [CoupledMCMC](https://github.com/nicfel/CoupledMCMC) can be added by using the `--mc3` option. This will add the default CoupledMCMC options which can then be configured at runtime with `-D`.

bash
Create dynamic MC3 XML
dynamic-beast --mc3 --outfile dynamic_mc3_hcv_coal.xml hcv_coal.xml
Configure MC3 with BEAST
beast -D 'mcmc.chains=4' dynamic_mc3_hcv_coal.xml

The full `id` of a parameter you'd like to set must be specified. This allows `dynamic-beast` to work with XML files that have multiple partitions.

Minor refactoring.

Changed the parsing so that defaults that are already set are skipped.

This the first working version of `dynamic-beast`.

- [x] Tests
- [x] Code coverage
- [x] ReadME