CHG: underlying partition container from list ->vector

[Craig44]

have hit very poor performance recently, so that has forced me to check handling agents as vectors. Basically less memory mangement, we put a bool on an agent and check whether it is a valid agent and we can overwrite invalid agents. This will allow for fast random element lookup which is critical for mortality processes.

[Craig44]

Have switched to vector based partition which has proved to be quicker than list for 3 area models and other models. So vector has become the master and the list implementation is a branch that will remain and by called on for high spatial resolution model. The merge made at commit 831a478