[Feature] make cellfinder main functions' arguments keyword-only

[alessandrofelder]

Is your feature request related to a problem? Please describe.
As first suggested by @matham in #542 (comment), calling the various "main" detect+classify functions, which have many arguments, with the arguments in the right order, is error-prone.

Describe the solution you'd like
Make the main functions' arguments keyword-only

[alessandrofelder]

I am generally supportive of this. My only reservation would be if this is likely to cause maintainers of other libraries lots of (short-term) pain, but I don't think this is case for cellfinder?

[adamltyson]

Also supportive. It should be an easy enough fix for others.