Added the following functions to BASHInitialisation.sh:
- checkReadOnlyVariables
- convertInput
- valueInArray
- variableExist

Also the file BASHExtra.sh is sourced if it exist, so extra
functionality can be loaded aautomatically.

Added the following functions to disk.sh:
- diskFree
- getDirTree
- getFunctionsFromFile
- includeFile

Also added some documentation to both files.