fix: Pad target region by `max_amplicon_length`

[msto]

Closes #16 Closes #17

This PR fixes the padding of the target region to be consistent with the original fgprimer implementation. (My bad 🙃 )

https://github.com/fulcrumgenomics/fgprimer/blob/6cf2542e927ced37dd0dce4c335de8dff07789c7/src/main/scala/com/fulcrumgenomics/primerdesign/primer3/Primer3.scala#L89-L93

Other changes

• Validate that the max amplicon length permits sufficient space around the target region for primers to be designed
• Renamed region to design_region, to better reflect the target/design region distinction
• Renamed _pad_target_region to _create_design_region, for the same reason, and clarified its documentation
• Added min_primer_length property to Primer3Parameters (max_primer_length already existed)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.37%. Comparing base (ed5c25c) to head (e91b176). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #23 +/- ## ========================================== + Coverage 97.35% 97.37% +0.01% ========================================== Files 25 25 Lines 1589 1597 +8 Branches 300 302 +2 ========================================== + Hits 1547 1555 +8 Misses 23 23 Partials 19 19 ```

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