major restructuring of pyslk modules

• adapted exception handling for errorcodes for exists and iscached (!49 (merged)).
• new module raw (will replace module pyslk) in future
• new module core
  • functions from parsers moved to core and renamed
  • new nice wrapper functions in core
  • function names in core are equal to slk/slk_helpers commands
• removed version checks and user warning when old slk/slk_helpers versions used