Lower Mem AGKM + Variable Len Seqlet ID + Initial Exemplar-Based Hit Scoring

Corresponds to PR #78

• AGKM embeddings are now the default (also avoids importing tensorflow unless user wants to use the previous gapped kmer embeddings)
• cap on the agkm embeddings size; reduces memory use
• Implementation of variable-length seqlet identification
• Initial implementation of exemplar-based hit-scoring