Rework examples

[MassimoCimmino]

The number of examples is increasing with the addition of new features. The already implemented examples serve different purposes : some demonstrate the use of features, and others demonstrate the accuracy or calculation speed of pygfunction.

This issue aims to rework examples and classify them into categories (i.e. subfolders), tentatively:

1. Getting started : A series of examples that start with the definition of a bore field, the calculation of g-functions, and simulation of a bore field (then may be the definition of a network and the simulation of fluid temperatures).
2. 'Advanced topics' : A series of examples that demonstrate the multiple ways that pygfunctions can accomplish tasks, like the different solvers, boundary conditions, different types of GHEs, fluids, etc.
3. Benchmarks : Examples aimed at quantifying the speed and accuracy of pygfunction.