Add in a threshold to eliminate species pairs not present in the lambda table

[AntObi]

Description

This PR fixes the issue with Doper where if two species are not observed to have a similarity indices, then they are assigned a default probability by the underlying CationMutator instance. This results in doping suggestions which are likely to be unfeasible. The fix works by getting that threshold probability value and eliminating possible dopant predictions below that threshold.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)

Reviewers

N/A

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] I have checked my code and corrected any misspellings

[codecov[bot]]

Codecov Report

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

Project coverage is 73.36%. Comparing base (2e2c720) to head (1971ac5). Report is 115 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #249 +/- ## ========================================== + Coverage 73.29% 73.36% +0.06% ========================================== Files 24 24 Lines 1996 2001 +5 ========================================== + Hits 1463 1468 +5 Misses 533 533 ```

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