[WIP] Relative time series

[jackparmer]

Adds relative time series support to Dataset and Ingest classes.

Can do things like:

from nominal import Ingest

dataset = Ingest().read(path = '/Users/jp/Desktop/penguins.csv', relative=True, ts_col='Sample Number')
dataset.relative # True
dataset.relative_units # 'SECONDS'
dataset.upload()

or

from nominal import Dataset

df = nm.data.penguins() # some polars dataframe
ds = Dataset(df, relative = True, ts_col = 'Sample Number')
ds.upload()