Software "layer"

[oesteban]

Both @mledesmacarbayo and @daducci have pointed out concerns about the use of software "layer". I split this issue in two parts:

• One part regards the use of "layer". Even though it is a rather extended concept in informatics (https://en.wikipedia.org/wiki/Layer_(object-oriented_design) and https://en.wikipedia.org/wiki/Multilayered_architecture), I think that the term will not be interpreted correctly by reviewers. Therefore, replace.
• Second part is dealing better with the concept of dataset and software. Almost all of you have commented about this. This will be handled in #12.