Add structural variant signature plot

[mdml]

Modeled off of Figure 4b in Nik-Zainal, et al. (Nature, 2016):

• Rows are signatures (as in SBS plot)
• Columns are structural variant types (deletions, tandem duplications, inversions, translocations)
• Two groups of columns: clustered and non-clustered
• X-axes are sizes
• Y-axes are counts/fractions/proportions