IndexError on get_dvh if using interpolation outside of dose grid

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Line 492 of dvhcalc.py raises an IndexError if get_interpolated_dose is called on a z with no dose. I injected a print statement to verify that it occurs on z values with no dose plane and that dose.GetDoseGrid(z) returns a 0 length array.

I edited get_interpolated_dose to just return dose.GetDoseGrid(z) in this situation, but that causes further issues.

dicompyler-core/dicompylercore/dvhcalc.py

Lines 490 to 492 in 84ed037

	d = dose.GetDoseGrid(z)
	extent_dose = d[extents[1]:extents[3],
	extents[0]:extents[2]] if len(extents) else d

from dicompylercore.dvhcalc import get_dvh
dvh = get_dvh(struct, dose, 6, interpolation_resolution=(0.5, 0.5))
Traceback (most recent call last):
  File "<input>", line 4, in <module>
  File "/Users/ninja/PycharmProjects/DVH-Analytics/venv/lib/python3.6/site-packages/dicompylercore/dvhcalc.py", line 90, in get_dvh
    callback)
  File "/Users/ninja/PycharmProjects/DVH-Analytics/venv/lib/python3.6/site-packages/dicompylercore/dvhcalc.py", line 210, in _calculate_dvh
    dose, z, interpolation_resolution, dgindexextents)
  File "/Users/ninja/PycharmProjects/DVH-Analytics/venv/lib/python3.6/site-packages/dicompylercore/dvhcalc.py", line 492, in get_interpolated_dose
    extents[0]:extents[2]] if len(extents) else d
IndexError: too many indices for array: array is 1-dimensional, but 2 were indexed

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duplicate of #215