Major refactoring

Hi guys,

After a long period of minor adjustments here and there, I am now embarking on a comprehensive refactoring of this package.

I am studying the package in depth, considering four key aspects.

# Operative considerations
I am highlighting some important comments that have never been resolved, for example, by systematically moving them into docstrings (in a "!!! note" field). I am also writing almost-certainly-correct improvements (e.g., substituting `::AbstractFloat` in a signature with `::U) where {U <: ...}` .

# Educational considerations
I am writing docstrings and annotating the obscure necromancy spells that await us. (just joking, this code rocks. Wish I was good like @giopaglia!!!)

# Structural considerations
I am tidying things when I can, for example, rearranging the order of `using`/`include`/`export`s.

# Style considerations
I am conforming the style to be more Julia-like. Even if we do not follow a rigid codestyle (maybe we should), I am just changing things that are obviously strange (e.g., crazy indentations, some monocharacter variables, etc.)

I am trying to write my commit messages as clearly as possible to allow you to code things more comfortably (see the pr associated with this issue).

I already started working on a personal fork, but I am just about to push everything into this issue's branch.