Fix: fe and clr not calculated if ae not included in intervals (#473)

[Gero1999]

Closes #473

Just worthy to mention that the unit tests we do for pk.calc function will never cover these cases. Maybe is worthy to consider doing at some point a test for pk.nca with a dataset that contains all columns and a for-loop checking all parameters independently.

[billdenney]

I like this simple fix. Can you please add a test for it with pk.nca()?

[billdenney]

When merged this will likely fix #376, too.

[Gero1999]

@billdenney I implemented the test and detected a similar problem for clr parameters, so I fixed them. However, there are other parameters that cannot be run with pk.nca directly for other reasons. If needed, I think we can address them independently in another issue/PR