Fix dicompyler on ubuntu

[pijaginw]

Due to problems with running dicompyler on Ubuntu we have prepared this pull request.
It fixes few things:

• data in DICOM is mostly in string format (maybe this is a change in pydicom?),
• NX library is no longer supported (replaced it with matplotlib.path),
• Scaling of doses can be calculated incorrectly. We recalculate the scaling factor so it doesn't generate IndexOutOfBound exception when accessing dvh array.

[bastula]

Thanks for the pull request, will take a look soon. I am currently working on dependency updates so that I can get dicompyler running on up-to-date software libraries.

[pijaginw]

Before we close this pull request we must fix rendering of DICOM pixel_array in 2D View. Currently, it renders the images using only two colors: black and white.

[bastula]

The DVH data extraction has been refactored into the separate dicompyler-core project. I don't think the out of bound issue should be present as that code is reworked completely.

I still think there are some valuable changes in this PR, but will need some time to look over them after the dependencyupdate branch (also see #103) has been merged into master.