Adding and subtracting Pyleoclim series

[CommonClimate]

Could be to compute spatial gradients, remove a forcing (e.g. scaled insolation) from a signal to highlight internal variability, or remove the thermal component from d18O to get d18O_sw, or compute a composite, remove the global seawater curve from from d18Osw

[CommonClimate]

No clear science use-case at the moment ; might revisit later in the context of funded grant