Updated copyright dates.

Updated copyright dates.

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.