Idea: Get BEAUti to load an operator log and use it to weight the operators

[rambaut]

For some parameters (e.g., substitution model parameters) we achieve very high ESSs but they also have a high cost (cause a complete recalculation of the data likelihood). The operator weights could be scaled so that they achieve the same ESS as the other parameters by being called less often and improve speed per ESS.

In the operators panel in BEAUti we could have the option to load an operator log and use it to re-weight the operators. The operator log would need to include additional information - the original weights and the ESS of each parameter (as the operator log is written at the end of a run, the log file could be analysed to give these). See #1187