Add LLOQ to sampling design and add trough as exposure metric

[dominic-irx]

This PR does two things:

1. Add LLOQ to the user-facing create_sampling_design function so that this can be passed into collect_tdms. Tests and documentation were changed accordingly.
2. Add troughs as a column to dose_update, as trough is another exposure metric that is commonly used. This is intended as a temporary fix until we have time to write a create_output_design function which would specify what exposure metrics to output and when.

[jasmineirx]

Overall looks really nice. Good catch on the LOQ.

I'm requesting some minor changes in documentation and testing, but I would also like to discuss generalizing this to allow calculation of all concentration metrics. For example, if you were targeting a Cmax (say for tobramycin or similar drug), is there a reason this function wouldn't work for that? Can we change the name to make it more generic?

[jasmineirx]

Do you have an example case you can add that tests the LOQ censoring happens appropriately with a well designed create_sampling_design?

[jasmineirx]

lgtm, with minor rewrite to ensure tests pass