DOC: Add a consistency section to the contributing Guidelines

[jucordero]

Based on issue #117, we should add instructions on how to be consistent with units, tests, inputs etc. in the documentation and the code.
Some of the modules are intrinsically defined to accept inputs with slightly different formats (e.g. wavenumbers in power spectrum modules), and the tests/examples are seen to use different values and input array sizes.

Expected behavior
A clear and consistent set of assumptions and input values for the different tests and examples.