Jianfeng/shift remove lowcoverage

@Elmira88 @anbangx 
@jakebiesinger maybe ? 

I fix the remove lowcoverage issues that remove the head kmer may lose the readSet. I passed the ReadSet to the neighbors to keep the original information. 

@jakebiesinger , in order to use negative offset, I did some changes to the offset bitcode. Specificlly, I use the 24th bit as the sign position, the package and unpackage method need to check the sign bit to set or return the offset field

@Elmira88 , if possible, you can also use this branch to test the new graph to see if the result may change ?