dev: simplify operations

[geritwagner]

colrev search could trigger load and prep. There is no alternative to triggering load and prep sequentially after search. In addition, we may remove the colrev retrieve high-level operation.

Similarly, colrev pdf-get should always trigger colrev pdf-prep.

This would result in a series of operations that would be more familiar with literature review authors:

colrev search
colrev dedupe
# optional: colrev prep-man
colrev prescreen
colrev pdfs
# optional: colrev pdf-get-man
# optional: colrev pdf-prep-man
colrev screen
colrev data