v0.5.4

dvhcalc

• Implemented DVH interpolation. (#39)
• Implemented optional user-specified structure thickness
  for DVH calculation.

dvh

• Fix a bug in absolute_volume if a DVH instance's volume units
  don't use default of Gy.
• Fix a bug in absolute_dose if a DVH instance's dose units don't
  use default of Gy. (#19)
• Support decimal values for volume constraints (i.e.V71.6).
• Support decimal values for dose constraints (i.e. D0.03cc).

dicomparser

• Ensure that Rx Dose from RT Plan is rounded instead of
  truncated.
• Account for holes and bifurcated structures for structure
  volume calculation.
• Implement structure volume calculation using Shapely. (#28)