(RasPi) Fixes local SDL version finder

-Now checks if the already installed version is the same as the
selected one
-Added check_for_dif_sdl function to meet the end above
-Added $RET_arg and $RET_pickedSDL to meet the end above
-$SDLVersion ===> $SDLFull
-Added $versionSDL (just has the version number)
-$alreadyInstalledSDL ===> $alreadyInstalledThisSDL
-Added $alreadyInstalledAnSDL
-Added missing $RET_success in function check_for_pi (doesn’t affect
functionality, so it still works as it should)
-Function requirements is only called when SDL is going to be installed
-Added & edited comments
Implementation of existing incomplete SDL install

-Implemented safety against the possibility of a local incomplete SDL
install (before running this script)
-Added function check_if_sdl_incomplete
-Added function remove_sdl
-Removed $incDir and $localIncDir (pretty useless)