Update error message not be confused with the Python SDF package

There are two sdf packages. One that ships with epoch  and should be used to read epoch sdf files. The other one ( https://pypi.org/project/SDF/ ) has nothing to do with epoch. Give a proper error message if the wrong one is installed.


https://github.com/skuschel/postpic/blob/7a7af8a875544323505c8c732f0c9e279d68b5db/postpic/datareader/epochsdf.py#L81