validate reso in convert_datetime_index, fixup fixture;

[MarcoGorelli]

closes LinkedEarth/paleoPandas#31

numbers look a bit more reasonable now :)

[CommonClimate]

Thanks for adding the check to s resolution. Hopefully this is the only place in the code where we need it? What about from_pandas()?

[MarcoGorelli]

that's a good point - I checked and from_pandas calls this method anyway, so I don't think it would be needed there too (can add if you prefer though)