Minor change in API

While writing the documentation, I found the use of the words ``intensity`` and ``coords`` and the use of the variable ``f`` for the intensity/values (a legacy from ``flux``) confusing. I would like to suggest the following change in convention:

* ``intensity`` -> ``value``
* ``fmin/fmax`` -> ``vmin/vmax``
* ``coords`` -> ``indices``

the reason for the first two is that when doing a PPP dendrogram on a density cube, density is neither a flux or intensity, so I find the use of the word ``value`` more general. Similarly, ``coords`` is confusing to me because if we ever want to use 'real' coordinates (e.g. position in PPP space or on the sky) that will be the natural word, whereas what we are returning right now really are indices.

Leaves would have a ``values`` attribute that would replace ``f``, so that one would then access ``leaf.indices`` and ``leaf.values`` rather than ``leaf.coords`` and ``leaf.f``.

What do other people think about this? I can implement the changes if we decide to do this.