Reducing max cluster size and adding a catch for small clusters

[htjb]

Solving #23 and #21.

Reduced the maximum cluster number to 20 and added a warning if cluster_number = 20 suggesting that the user should explore their data a bit more externally to margarine.

Added a catch that lowers the cluster number if any of the clusters are smaller than 100 samples. Prevents errors when we try to train/test split clusters with fewer than 5 samples and ensures that the test size for each cluster is somewhat reasonable (minimum 20 samples).

[htjb]

Need a test for small clusters!

[codecov-commenter]

Codecov Report

Patch coverage: 94.44% and project coverage change: +0.16 :tada:

Comparison is base (35243df) 88.76% compared to head (46fb86b) 88.93%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #24 +/- ## ========================================== + Coverage 88.76% 88.93% +0.16% ========================================== Files 4 4 Lines 454 470 +16 ========================================== + Hits 403 418 +15 - Misses 51 52 +1 ``` | [Impacted Files](https://app.codecov.io/gh/htjb/margarine/pull/24?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Harry+Bevins) | Coverage Δ | | |---|---|---| | [margarine/maf.py](https://app.codecov.io/gh/htjb/margarine/pull/24?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Harry+Bevins#diff-bWFyZ2FyaW5lL21hZi5weQ==) | `95.95% <94.44%> (-0.14%)` | :arrow_down: |

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