Allow to generate largest probe possible when not enough oligos are found

[ggirelli]

Is your feature request related to a problem? Please describe.
If not enough oligos are present in the region of interest, the script stops.

Describe the solution you'd like
Generate the largest probe possible. Of course, stop if no oligos are found. Also, include a script option to roll back to the old behavior of stopping if not enough oligos are present, for scenarios where the number of oligos needs to match exactly the requested number.

[ggirelli]

Implemented in v2.0.4 (to be released). Use --exact-n-oligo to revert to previous behavior (i.e., raises AssertError).