Ignition Math Python interface needs to export PYTHONPATH

[ahcorde]

When SWIG is active and we are building Ignition Math Python interfaces the PYTHONPATH environment variables is not been setted in the console. We should do it manually.

I checked ign-cmake and I didn't find any function that provides this functionality.

[chapulina]

In which situations is PYTHONPATH needed?

1. When compiling ign-math
2. When compiling downstream SWIG bindings that use ign-math's bindings
3. At run time

[ahcorde]

3. At run time

[chapulina]

Here's an issue that's also talking about the ability of setting environment variables at runtime: gazebosim/gz-cmake#185

I think at the moment the best we can do is install a setup.sh file and instruct users to source it before using the library.

[chapulina]

See #271