global: PEP-8 style in block comments

• Amends block comments in all files to replace leading ## by # in order to be better PEP-8 compliant, in leading copyright message blocks but also elsewhere. (addresses #1904)

• Leaves triple and other multiple leading comment marks alone for future ad-hoc amendments.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>