create appveyor config

[orbeckst]

In order to support native MDAnalysis windows builds (see ge PR MDAnalysis/mdanalysis#1880) we also need a gridDataFormats native windows package.

The appveyor https://ci.appveyor.com/project/orbeckst/griddataformats project is enabled but currently fails.

[orbeckst]

@tylerjereddy would you be able to help with getting GridDataFormats running on AppVeyor?

[orbeckst]

Actually, as a pure Python package, pip install on windows should be fine. Still needs to be tested, though.

@tylerjereddy a quick look from you would be highly appreciated.

[kain88-de]

we are distributing this package on conda-forge for windows. Pretty much since ever.

https://anaconda.org/conda-forge/griddataformats/files