Scdiff

Added more visualization functions:
1) Integrated PANTHER and Toppgene GO analysis
2) Added more 2D visualizations of the scRNA data (PCA, t-SNA ISOMAP)
3) reduce the DE fold change cutoff from 1.5 to 1.0

Updates:
* added TSNE plots of the cells
* added PCA plots of the cells
* added ISOMAP plots of the cells
* added -f parameter
* added functional analysis of the differential genes
* added the function to find differential genes between two specified nodes
* added the function to download the json file, which contains all information of the resulting model

updates:
Updated Virtual Ancestor option
Added format error detection for input config file.

In this update:
fixed the python3 compatibility issue.
fixed the global variable issue
updated the scidff.viz module