change to 2-clause BSD

unmarked as a Work In Progress

@torosdahl @dvarjas following a suggestion of @cwg I propose to change to 2-clause BSD (the removed clause is rather irrelevant)

All good with me.

added 9 commits

• 8136747b - fix whitespace
• d5a14fab - clean up changelog
• 89ea1351 - bump copyright year
• 1499c9f1 - remove erroneous export 'kwant_continuum'
• cc6bc81f - remove parentheses in class definition
• b6d1e899 - fixup docstrings
• 0aa3e157 - whitespace
• c428c4ca - simplify check for Hermiticity/antisymmetry
• f5c1a95d - change to 2-clause BSD

Compare with previous version

merged