Add dm4 reading capabilities

I have implemented dm4 reading but I don't have any dm4 that contains experimental data—I have used the free version of DM to create "synthetic" test files. Could someone test the new reader with "real" dm4 files?

Also now there is an easy way to map attributes from original_parameters to mapped_parameters (have a look at  https://github.com/francisco-dlp/hyperspy/blob/ENH_dm4/hyperspy/io_plugins/digital_micrograph.py#L722). I have mapped only 4 parameters, mainly EELS ones. Could you think of any other parameters that we should map?

Thanks!

Francisco