Add explicit testing for hdf5dll.dll on Windows to improve diagnostics

[mwiebe]

Ran into this because of a bug in the HDF5 windows installer.

[avalentino]

Sorry @mwiebe, I'm not an expert of ctypes, but it is not enough just using something like

if not ctypes.util.find_libraty('hdf5dll.dll'):
    raise ImportError('Could not load "hdf5dll.dll", please ensure'
                              ' that it can be found in the system path')

or

try:
    l = ctypes.CDLL('hdf5dll.dll')
except OSError:
    raise ImportError('Could not load "hdf5dll.dll", please ensure'
                              ' that it can be found in the system path')
else:
    del l

?

[mwiebe]

Yeah, those probably would work too, and be simpler. I'm not really an expert on ctypes either.

[scopatz]

Either way, I think that we should merge this fix in. @avalentino's is cleaner and would test it if I had a windows machine....

[avalentino]

@mwiebe can you please confirm if at least one of the simplified version of the patch works for you?

[mwiebe]

I've switched it to use the find_library function, which works on my machine. I also modified setup.py to automatically find the HDF5 directory if it's in the PATH, because I don't like having to set that manually.

[scopatz]

@mwiebe Much agreed. This looks good to me now. @avalentino Do you want to merge it in or should I?

[avalentino]

Done. It is now in the develop branch.
Thanks again @mwiebe