Import metrics python3 compatible

[Nozdi]

Hey there,

I wanted to use your code to compute surface distance but one import was not python3 compatible.
Here is a PR which sorts out this issue.

Cheers

[ntomita]

Hi @derpson, this PR is important for those who use python3. I also confirmed this change works. Please review and merge this. Thanks!

[zzzzzzzzzzzzz]

Guys, please merge it. Faced the same issue, though fixed it manually

[SimonBiggs]

We're planning on making use of this in an upcoming project, and we are interested in the long term maintenance of it. It appears that for the time being this package isn't a priority of Deepmind's (I mean they have buckets of other things, so who can blame them. I am grateful that it has been made open source).

Would others here be interested in having this library being made available on PyPI by being made available within the following PyPI package that I maintain (with this fix of course)?

• https://pypi.org/project/pymedphys/

A good location for it I believe would be alongside the DICOM structure tools:

• https://github.com/pymedphys/pymedphys/blob/daab309f2690c1f7292ddc4ccbcfe9670eb8ff22/pymedphys/_dicom/structure.py