Remove extraneous functions from export list

[mtfishman]

Before the release, we should go through exports.jl and remove anything that we don't want to be public functions, such as:

• [x] choi
• [x] choimatrix
• [x] readouterror!
• [x] projectchoi
• [x] convertdatapoint
• [x] gradlogZ
• [ ] nll
• [x] gradnll
• [x] gradients
• [x] fullfidelity

It is best to keep this list as short as possible, especially at the beginning.

[GTorlai]

I'm doing this already in the current PR