Re-use pyleoclim infrastructure

[CommonClimate]

Currently, cfr only uses pyleoclim for spectral analysis. However, the ProxyRecord class is basically GeoSeries; only "seasonality' is missing.

Making ProxyRecord a child of GeoSeries would enable Pyleoclim functionalities, particularly:

• pandas conversion with to_pandas()/from_pandas()
• time unit conversions
• standardize()
• center()
• bin()
• gaussianize()
• and maybe more

[CommonClimate]

would be easier to do with lightweight pyleoclim package; see this issue