Simplified ColumnMapper names as properties of the ensemble

[nevencaplar]

When working with the ensemble I often felt it would be natural to call columns which were explicitly defined in ColumnMapper. For instance, if I used

col_map = ColumnMapper(id_col="SDSS_NAME_dr16q_constant", time_col="mjd_ztf_source", flux_col="mag_ztf_source", err_col="magerr_ztf_source", band_col="band_ztf_source")

it seems natural to use something like ensemble.source[flux] instead of ensemble.source[mag_ztf_source]. Now, that is not possible, but I suggest to assign the simplified column names as property of the ensemble, e.g., ensemble.source.flux to get the same result as generated by ensemble.source[mag_ztf_source] now.

[dougbrn]

If the desired usage pattern is ensemble.source.flux, this will be blocked by the refactor (#191 ), as it requires the ability to add properties to the underlying dataframe itself. Which is dask currently, but will be our own SourceFrame object after the refactor.

And just to be clear, these properties should be distinct from the current ensemble._flux_col, etc properties. Where ensemble.source.flux returns a series, and ensemble._flux_col returns the string column name.