clean.conc.blq: feats for tmax reference and default keep

[Gero1999]

Hello, thank you for this great package! I would like to propose two potential features for this function:

• Add before.tmax & after.tmax in conc.blq list argument. In NCA is very typical to use tmax as a reference on how to deal with BLQ data. I think that it would be very easy to change PKNCA clean.conc.blq to include for the list definition "before.tmax"and "after.tmax" to make this possible, but please let me know what you think!

• Default keep values in conc.blq list argument . I feel that when a user does not define a specific scenario in the list argument (i.e, middle) they simply don't want to perform any action. For that reason, I believe it would make sense to change PKNCA.options to put keep as default when an element list is missing.

Let me know what you think of these requests, I will also try to link a PR with a potential solution. Feel free to give it a look, change or ignore it!

Pull request in #341

[billdenney]

Thanks for the suggestion. It makes sense to me. I've started an issue (#343) for discussion of the implementation.