Please add option to record the ultimate location of the "CALIB" symb. link

We use root/CALIB as a convenient way of ensuring that everyone is using the latest set of calibrations. Unfortunately, this loses provenance information.

Please add a metadata keyword CALIBROOT to the calexp outputs.

I note that this is not a good long-term solution – we defer that to the gen3 middleware – but it's a pragmatic way for PFS to make progress. As such, it's probably not worth saving the same metadata to other files (e.g. pfsArm).