Upgrade and extension to include FEI/ThermoFisher images

The code has been modified so that the entire header cna now be reliably
extracted and so that selected parameters can be reliably extracted.

Further functionality has been added to handle images from the xT
software from Thermo Fisher Scientific (FEI).

Parameters can now be dumped to json as well as printed or further
used in code.

Electron channeling patterns are also handled for ThermoFisher
images when calling get_image_pixel_size() - here the image
is calibrated in angle rather than distance. The correct value
and units are returned from the code.