Upgraded all VISAN source code to be compatible with Python 3, wxPython 4,
and VTK 8.
VISAN is no longer a self contained application but can only be installed
in a conda environment (using e.g. Anaconda or Miniconda).
External dependencies of VISAN such as wxPython and VTK, but also CODA and
HARP are no longer included with VISAN, but will now be automatically
installed into the VISAN conda environment as external packages.
This approach now also allows for easy addition of other python packages to
be used in VISAN by installing them in the same conda environment.
Changed Open Source licence of VISAN from GPLv2 to BSD (3-Clause).
Updated VISAN icon and splash screen based on new Atmospheric Toolbox
To install VISAN use the conda package