External module imports

[nichollsh]

The code currently as utils.modules_ext.py to import external modules such as Numpy, Scipy, Matplotlib, etc. This means that modules are sometimes imported when not required, and also makes code linting very tricky.

Instead, modules should be imported within the files in which they are used.

[nichollsh]

@stefsmeets I think this is connected with your Issue #131 and the code structure reorganisation into the usual package format?

[lsoucasse]

Yes! we discussed it yesterday with Stef.