Add vasprun parsing for split k-point paths

[brlec]

Added a DataVasprun class in inputs to parse data directly from a vasprun.xml file. It can parse individual vaspruns or vaspruns from split k-point paths i.e. as generated with sumo for hybrid calculations like shown below:

band/              
  ├── split-01/    
  ├── split-02/
  ├── split-03/
  └── split-04/
    └── vasprun.xml

DataVasprun only needs one argument - path to files. If we are interested in parsing only one specific vasprun that contains the entire k-point path, path=path/to/vasprun.xml. However, in the case of multiple folders with separate vaspruns, such as above, path=band will parse all the subfolders and vaspruns from there. Specifically this is made to work with sumo formatting but it should be easy enough to adapt to any other formats.

[lucydot]

Hi Katarina (@brlec), thanks for contributing this code. It looks good - before merging with master I'll write some tests and push to your fork of effmass. I'll probably do this next week (of course feel free to do this yourself if you want, just let me know if you're working on it here). Thanks! Lucy