Various column names

Clump file columns:
    "index", "lev", "parent", "ncell", "peak_x", "peak_y", "peak_z", "rho-", "rho+", "rho_av", "mass_cl", "relevance"

Mergertree file columns:
    "clump", "progenitor", "prog outputnr", "desc mass", "desc npart", "desc x", "desc y", "desc z", "desc vx", "desc vy", "desc vz"


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The merger trees are stored in `output_XXXXX/mergertree_XXXXX.txtYYYYY` files. Each file contains 11 columns:

* `clump`:            clump ID of a clump at this output number
* `progenitor`:       the progenitor clump ID in output number "prog_outputnr"
* `prog_outputnr`:    the output number of when the progenitor was an alive clump
* `desc mass`:        mass of the current clump. 
* `desc npart`:       number of particles of the current clump. 
* `desc x,y,z`:       x, y, z position of current clump.
* `desc vx, vy, vz`:  x, y, z velocities of current clump.

desc_mass and desc_npart will be either inclusive or exclusive, depending on how you set the `use_exclusive_mass` parameter. 
(See below for details)

**How to read the output:**

* A clump > 0 has progenitor > 0: Standard case. A direct progenitor from the adjacent previous snapshot was identified for this clump.
* A clump > 0 has progenitor = 0: no progenitor could be established and the clump is treated as newly formed.
* A clump > 0 has progenitor < 0: it means that no direct progenitor could be found in the adjacent previous snapshot, but a progenitor was identified from an earlier, non-adjacent snapshot.
* A clump < 0 has progenitor > 0: this progenitor merged into this clump, but is not this clump's main progenitor.
* A clump < 0 has progenitor < 0: this shouldn't happen.

### Visualisation

`ramses/utils/py/mergertreeplot.py` is a python 2 script to plot the merger trees as found by this patch. 
 Details on options and usage are at the start of the script as a comment.