Allow install into conda env with GitHub and pip

[craig-warren]

I wanted to be able to install PyEVTK from GitHub into a conda environment through pip in a conda environment.yml. This was not possible because of the way the version number was being imported. So these changes now permit a conda environment.yml to look like, e.g.

name: my-env
dependencies:
- python>3.6
- jupyter
- matplotlib
- numpy
- pip
- scipy

- pip:
  - git+https://github.com/paulo-herrera/PyEVTK.git