Reorganize GSEA function

[yihming]

Rename fgsea function to gsea, and modify:

• Add method parameter: default is "gseapy"; alternatively support "fgsea"
• For GSEAPy method, use gseapy.prerank function, and rename columns in resulting data frame to match fgsea, so that plot_gsea function and write_gsea_results_to_excel function can work properly
• Combine de_key and log2fc_key to rank_key, which takes "de_key:attr_key" format so that users don't need to specify 2 parameters
• Set n_jobs=4 by default
• Only use genes with non-NaN q-values for GSEA
• Add verbose parameter for GSEAPy method
• GSEA results are sorted by padj values in ascending order, then by absolute values of NES scores in descending order, and then by pathway names in dictionary order

Rename write_fgsea_results_to_excel to write_gsea_results_to_excel