1. Fix misleading indentation warnings.
2. Fix creating a temporary const copy when iterating over a map (thanks to range analysis warnings)
3. Fix creating a copy when iterating over a range without reference qualifier (also thanks to range analysis warnings)
Changed copyright end year to 2020 in every reference "Copyright (c) 2006-XXXX, assimp team".
Changed copyright end year to 2020 in every reference "Copyright (c) 2006-XXXX, ASSIMP Development Team".
Changed copyright end year to 2020 in LICENCE.rtf.
Changed copyright end year in CMakeFiles.txt files and any other places referencing Assimp with a copyright start and end year.