Simplify process_I()

[thibautjombart]

This should be done to adapt to the fact that the basic input to estimate_R and wallinga_teunis() are now numeric.
To do once #200 and #152 are closed.

[sangeetabhatia03]

@thibautjombart
@rebeccanash flagging that I am refactoring process_I (#242 ); I can fold any proposed changes into that issue - I'll check with you both.

[sangeetabhatia03]

hi @rebeccanash and @thibautjombart when you have a chance, can you take a look at the changes in PR #248 and let me know if you wanted any other changes.