Make default dataset for `n_obs` and `n_obs_1`

[zachary-foster]

When plotting and filtering, it is often useful to plot the number of observations for each taxon using n_obs or n_obs_1. However, since these require a dataset to be specified, they are not in the NSE list. If a default is added, then they could be used with NSE. Either:

1. If there is one dataset, use that, otherwise error

or

2. Always use the first dataset, error if there are none.