Moved (re)loading of user settings library to vpp43 module.
Adjusted imports and exported symbols.
Changed function in atextit so a reference to closing function is kept and GC is avoided.
Adjusted usage of partial function.
Improved wrapper.functions

- Reorganized imports.
- Added a list as an attribute named `_functions`
  to CDLL or WINDLL to register specified functions.
- Corrected `uninstall_handler`.
Reworked calling to parent class methods in errors module.
Renamed visa module to legacy.
Adjusted headers.
Added infor to UnknownHandler error
Added conversion string / bytes / string for visa functions.
Added high level module.
Renamed wrapper to ctwrapper
Moved legacy code to legacy module
Changed sphinx theme to readthedocs