Reorganize GSEA functions

[yihming]

Rename fgsea function to gsea, and modify:

• Add method parameter: default is "blitzgsea"; alternatively support "fgsea"
• Set min_size to 5, and max_size to 4000
• For BlitzGSEA method, set symmetric=False, permutations=2000 and center=False by default
• For fgsea method, rename columns in resulting data frame to match BlitzGSEA result, so that its output could also be used to generate BlitzGSEA plots
• Only use genes with non-NaN q-values for GSEA
• In GSEA results, ignore terms with -Inf or Inf NES scores.

Rename write_fgsea_results_to_excel to write_gsea_results_to_excel