pyMOAB not supported on Ubuntu 20.04 with python3

[Kalin Kiesling]

Ubuntu 20.04 comes with only python3. This means that there is only one python executable as python3 and no python or python2 executables. When installing MOAB with pyMOAB enabled, it fails with this error: `/moab/build/pymoab/install.sh: line 11: python: command not found`

That line can be found here.

One suggested fix is to set the python executable to `python3` if python does not exist:

if [ -x "$(command -v python)" ]; then
  python_exe=python
elif [ -x "$(command -v python3)" ]; then
  python_exe=python3
fi

$python_exe setup.py install --prefix=@CMAKE_INSTALL_PREFIX@ --record @PYMOAB_INSTALL_PREFIX@/install_files.txt

‌

[Vijay M]

PR #494 addresses this issue and the fix is now merged to master.