Added pyyaml as a dependency because the BigDFT parser needs it. I don't think...

Added pyyaml as a dependency because the BigDFT parser needs it. I don't think there currently is a parser specific way to install dependencies, so I added it here.

requirements.txt

 future
-numpy 
+numpy
 cython
 Pint==0.7.2
 ase==3.12.0
@@ -7,3 +7,4 @@ setuptools
 scipy
 mdtraj==1.7.2
 pandas
+pyyaml