ytakemon/GRETTA: v0.6.0

Version 0.5.0 was submitted to Bioinformatics and version 0.6.0 addresses several reviewer comments (detailed below). GRETTA is now GRETTA (with two T's) due conflict with existing package on CRAN. Upon loading GRETTA into R, a welcome message is displayed to indicate package version and the latest DepMap data set that is available. Function arguments are now all lower case. GI_screen() has a new argument gene_list = to allow small-scale GI screens and reduce computational time. Default pan-cancer coessential_map() now uses pre-computed cor matrix to reduce computational time. This matrix is provided along with the DepMap data. A vignette is now available and in a Bioconductor format to prepare for submission (same as readme). Examples are now mostly run-able to comply with Bioconductor