Rename R2s parameter in hmri_defaults

[Barisevrenugur]

(after discussion with @lukeje ) Due to multiple different R2* estimation algorithms (e.g. OLS, WLS and NLLS), drop the name-dependence of the parameter on a specific method (i.e. OLS): rename the parameter on line LineToBeChanged possibly to hmri_def.R2estatics and implement the required changes in hmri_create_MTPROT.m

[lukeje]

One point which we will need to bear in mind is that changing parameter names in the defaults file will break backwards compatibility. We will thus need to make this change very clear to users downloading the new version. As far as I know, though, people don't normally disable ESTATICS, so we should be OK in most cases.

[lukeje]

Also, I realised the context is missing. We have a boolean parameter which for historical reasons is called R2sOLS to turn on ESTATICS. However this is confusing now that we have multiple different R2* estimation algorithms (e.g. OLS, WLS and NLLS). It would therefore be a good idea to rename the parameter to turn on ESTATICS to something less confusing.