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
EHN: tidy up utils utils.eds
Merge branch 'master' of https://github.com/hyperspy/hyperspy into tools_for_bulk
DOC: slight changes
Merge branch 'master' into tools_for_bulk
Some small style and typo corrections
Rename density_from_composition -> density_of_solution
Rename rho -> density
Move the units_converter to utils/material/composition and rename density_from_composition to density_of_mixture and move it to utils.material.composition
Split test_xrange in test_xrange and test_electron_range, and avoid using density_of_mixture
Make weight_to_atomic and atomic_to_weight return the percentage. Add tests for these two functions and add utils to setup.py
Rename compo_at and compo_wt to atomic_percent and weight_percent
Rename density_of_mixture -> density_of_mixture_of_pure_elements
BUG-FIX: density_from_mixture_of_pure_elements was not returning the correct result. Add test and remove the option to give the composition in atomic_percient
Vectorize functions in utils.material.
Move utils subpackes to modules of the same name.
Merge pull request #108 from hyperspy/tools_for_bulk

EHN: New physics tools for bulk sample