Provide a Geometry Optimization how-to

While the tutorial is OK, an additional Geometry optimization how-to would be nice.

It should include:

1. What options are relevant for FIRE, Broyden and CG?
2. What systems can we expect to work better with each of them?
3. Clarification on how the constraints work.
4. Clarifications on the Zmatrix format.

A few examples of these:

• Sections of the output to check (enth, Voigt, outcoor, outcell) and other relevant files (FORCE_STRESS, FAC, FA).
• Constraints cannot be used to enforce symmetry, one cannot fix all vector lengths or all angles at the same time.
• ZMatrix is the only way to add angle or bond constraints. Add that siesta does not output a resulting Zmatrix.

I don't think quenched MD should be included here; while it is a relaxation method, it is not a minimization per se.