[IMP-dev] patches to embed

Here is a patch with various important and minor patches to embed. These include - change some of the io errors to throw exceptions rather than return error codes as the error codes pretty much never checked in EMbed and don't propagate out. It also checks error codes more often for good measure) - write error messages to stderr instead of standard output - add a method to access a trilinearly interpolated density (so that the returned values are continuous) - initialize the data in density maps to 0 on construction - add a version of get_origin with takes an int (like the get-max) - add a few more error checks - tweak the endian hack in MRCReader since it was not getting the endianness right on my files.