Add element check + new metallic scheme

[akvatol]

Add options to check elements consistency for structures

[pep8speaks]

Hello @akvatol! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file scripts/aiida_submit_ext_scheme.py:

Line 14:80: E501 line too long (88 > 79 characters)
Line 18:80: E501 line too long (85 > 79 characters)
Line 19:80: E501 line too long (86 > 79 characters)
Line 51:80: E501 line too long (83 > 79 characters)