Interval method

There are actually three fixes here:
1. Fix bug to allow the flip_sign option to work correctly in hybrid_plot
2. Fix bad behavior when working with sparse data from bigWig files. The default value when no statistical summary is returned for an interval is now undef instead of 0, allowing that interval or pixel to be skipped and a line extended to the next defined value, rather than jumping to 0. This is the same behavior as with binary wiggle (wib) files.
3. Added the interval_method option when working with statistical summary (bigWig) data so that min, max, and sum values may be graphed as an alternative to the default mean value. The sum works quite well when graphing densities of SNP or CpG features from a bigBed file.