Set a user functions path

[DougRzz]

I would like the users of my Vaex based app to be able to create their own NumPy/Pandas type functions similar to functions.py. To enable this, I would like to be able to specify a list of paths which contain these additional functions.

Maybe the api could be something like this:

vaex.userFunctionPaths = [r'C:\Dougs\vaexFunctions\DougsFunctions.py',  r'X:\team\functions\extraFunctions.py']

[maartenbreddels]

I guess you mean something that runs when you import vaex?
It does currently https://github.com/vaexio/vaex/blob/master/packages/vaex-core/vaex/__init__.py#L550
But it's a bit of a leftover artifact, but does it suit your needs?

[DougRzz]

Something similar to that would work maybe. I would like the user to be able to write and save their own functions and then add them to the expression_namespace without having to modify functions.py in the Vaex code base.

[DougRzz]

This PR fulfils this issue.
#188
:)