*.mrc compatibility when not using denoiser #45

Originally reported in #44.

When running partinet detect with a directory of .mrc (32-bit or 16bit) micrographs without the denoiser stage, the command fails with an AssertionError because the underlying DynamicDet network only supports standard image/video formats (jpg, png, tif, etc.):

AssertionError: No images or videos found in /scratch/partinet_test/motion_corrected. Supported formats are:
images: ['bmp', 'jpg', 'jpeg', 'png', 'tif', 'tiff', 'dng', 'webp', 'mpo']

Current workaround: batch process micrographs using EMAN2: e2proc2d.py *.mrc @.png
Fix: Add .mrc to the list of supported file types and implement loading via a library such as mrcfile, normalising the 32-bit/16 bit float data to an 8-bit or 16-bit image array compatible with DynamicDet.