EHN: New physics tools for bulk sample

X-ray and electron range. Density from composition.
DOC: add range in doc
EHN: add tests
EHN: clearer variables
BUG-FIX: EELSModel.resolve_fine_structure was raising an exception when there are not edges in the model
BUG-FIX: EELSModel self.edges was undefined in an empty model
EHN: tidy up utils utils.eds
Merge branch 'master' of https://github.com/hyperspy/hyperspy into tools_for_bulk
DOC: slight changes
Merge branch 'tools_for_bulk' into edslab

Conflicts:
	hyperspy/_signals/eds.py
	hyperspy/misc/eds/__init__.py
	hyperspy/misc/eds/utils.py
	hyperspy/tests/signal/test_eds_sem.py
Merge branch 'master' of https://github.com/hyperspy/hyperspy
EHN: right plotting for bins='blocks'

unequal bins width supported only for plotting
Merge branch 'master' into edslab
Merge branch 'get_histogram' into edslab