Allow mapWig & repTimeWig to be optional

[ycl6]

This PR make changes to the run_ichorCNA and wigToGRanges functions, so that users can skip supplying the two optional wig params (mapWig and repTimeWig) when running ichorCNA.

Error in wigToGRanges(mapWig) : 
  argument "mapWig" is missing, with no default
In addition: Warning message:
In message("wigToGRanges: WIG file '", wigfile, "' not found.") :
  restarting interrupted promise evaluation

Error in wigToGRanges(repTimeWig) : 
  argument "repTimeWig" is missing, with no default
In addition: Warning message:
In message("wigToGRanges: WIG file '", wigfile, "' not found.") :
  restarting interrupted promise evaluation

I also added checks to make sure the two required params tumor_wig and gcWig are provided by users and improved the error messaging.

Previously:

Error in substr(tumor_wig, nchar(tumor_wig) - 2, nchar(tumor_wig)) : 
  argument "tumor_wig" is missing, with no default

Reading GC and mappability files
Error in wigToGRanges(gcWig) : 
  argument "gcWig" is missing, with no default
In addition: Warning message:
In message("wigToGRanges: WIG file '", wigfile, "' not found.") :
  restarting interrupted promise evaluation

Now

Error in run_ichorCNA(id = "test", ...
  tumor wig file not provided but is required

Error in run_ichorCNA(id = "test", ...
  GC wig file not provided but is required