site_parameters() should allow users to set a parameter draw

[tbreweric]

The site_parameters() would benefit from an additional argument (e.g. parameter_draw) which allows users to change the parameter set they are using when setting up malariasimulation parameter lists.

A proposed update has been written in the branch parameter_draw and a PR (#41 ) is open to merge the change.

The PR is currently paused awaiting a possible malariasimulation update which would allow us to integrate this feature more easily. This update is addressed in the following malariasimulation PR.

[tbreweric]

The malariasimulation PR has now been merged to the dev branch. When the next malariasimulation update occurs we can revisit this PR.