Make cluster swap relative threshold a configurable parameter

[ppebay]

As suggested by @lifflander in PR #433 , the 5% value in line 117 0f Execution/lbsClusteringTransferStrategy.py should be made an optional (with default value of 0.05) user-defined parameter

A suggested name is cluster_swap_rtol.

Please make sure that CI is updated accordingly and that this new configuration parameter is tested for invalid types.

[ppebay]

Capturing gist of the discussion to summarize how this would ideally work:

The 'InformAndTransfer` algorithm, line 60, fetches the name of the transfer strategy:

strat_name = parameters.get("transfer_strategy")

This is where, possibly, the Clustering transfer strategy might be chosen. For instance, in config/challenging-toy-fewer-tasks.yaml:

transfer_strategy: Clustering

So what we would want there, would be an additional, optional parameter called cluster_swap_rtol.

Ideally, CI would test for the consistency of it all (in a fashion similar to what it's already doing for other parameters, both required and optional).

@cwschilly