Read HDF5 enumeration datatype as integer

In molecular simulations I use the HDF5 enumeration datatype to store particle species in an [H5MD](http://nongnu.org/h5md/h5md.html) file. [HDF5 1.8.11](http://www.hdfgroup.org/HDF5/doc/ADGuide/Changes.html#1811) or greater supports implicit conversion from enumeration file datatypes to integer memory datatypes, which permits us to read at least the enum values.