BUG: axis names for multiple stack changed

from -i to _i
Merge pull request #113 from hyperspy/bug_in_stack

BUG: axis names for multiple stack changed
Merge branch 'master' into fix_kka
Merge branch 'fix_kka' of https://github.com/AEljarrat/hyperspy into fix_kka
BUG-FIX: some KK tests were broken
Add DielectricFunction signal, rename KK to kramers_kronig_analysis, modify the tests accordingly and add user guide test. This is a partial commit, KK is not yet fully working.
Crop the energy axis at 0. + 1 and correct the tests
Correct docs.
Merge pull request #1 from francisco-dlp/fix_kka

Merge changes from Francisco