Add examples of histogram configuration, with and without explicit grid parameters

[giacomofiorin]

As the title says. Besides providing a useful example of the histogram itself, there is also a motivation to encourage users to use histogramGrid to avoid conflicts between multiple uses of width. The following is a situation (quoting @jhenin) where the role of the width keyword becomes cumbersome:
1) I want a harmonic restraint. I set the force constant to a value I would write in a paper, in kcal/mol/A^2, I leave width at default.
2) I add a histogram block. For that I need to set width to some value different from 1.
3) I need to change the value of the force constant in the harmonic block accordingly. I find this last step more complicated than necessary.

Thanks to the addition of the histogramGrid { ... } option in 2016, the above situation does not need to occur. But it requires reading the documentation to a deeper-than-average level of detail, which is unlikely to occur in such a situation. Hence the second example.

Both examples also showcase the auto-detection of variables' boundaries, added earlier. I'm fine if people want to re-type those boundaries explicitly, but they should be aware of the auto-detection feature (which was necessary to remedy problems raised for metadynamics by the use of conventions adopted earlier in pre-Colvars ABF implementations).

On the subject of allowing custom grid parameters for biases (discussed earlier), I'm in favor of it and the core of the implementation is straightforward, just using colvargrid<>::parse_params(). @jhenin I can add the feature and the tests very quickly if you volunteer to document it.

[jhenin]

@giacomofiorin Thanks, I definitely agree on generalizing this to grid-based biases in general. I can do the documentation.

[jhenin]

Why is this block commented out?

[giacomofiorin]

It's a file that I accidentally committed before it was finished, but it's not used yet. I wanted first to simplify a bit the generation of the backend tests, but it's probably easier to still add it manually.