Erroring out appropriately when _read_openjpeg gets differing sampling

If the subsampling factors are different, we should error out when using
read with libopenjpeg.  Closed #72
Got rid of read_bands tests.  Reasserting ValueError tests.

read_band is intentionally unimplemented for libopenjpeg.  The two
ValueError tests originally were cast as RuntimeError tests, as
libopenjp2 catches this.  Libopenjpeg cannot catch it as easily.