feat: EFO l2g merge features

[xyg123]

✨ Context

This PR implements the merging of features for studylocus-to-gene in L2G, this is to address the high rate of missingness as well as the class imbalance between Goldstandard positives/negatives.

🛠 What does this PR implement

New function in the L2GFeatureMatrix class: "merge_features_in_efo":
For a given studylocus-to-gene entry in the L2G feature matrix, say studyLocus_1 to gene_1 :
Identify all other studies which are of the same EFO/trait.
Identify other studylocus-to-gene entries for gene_1 from these studies, which are within a set window size (default 500kb) to studyLocus_1:
merge all features from these entries together into a single row, taking the maximum where necessary.

New function in the L2GGoldstandards class: "balance_classes"
Downsamples the Goldstandard negative set randomly, based on a predefined upper limit for the GSN:GSP ratio (defaults to 2).

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