Allow custom hyperparameters be passed into somaticseq_parallel.py

Allow xgboost hyperparameters be passed into somaticseq_parallel.py, e.g., somaticseq_parallel.py --somaticseq-train --extra-hyperparameters scale_pos_weight:0.1 seed:100. Previously, they could only be passed into somatic_xgboost.py. Beware, however, multi-argument options like --extra-hyperparameters and --features-excluded cannot be placed immediately before paired or single, because otherwise it'll try to include paired or single as an argument instead of invoking paired or single mode.