Add output column headers to .forward files

[RoryBarnes]

Gijs Mulders suggested that it would be useful to include commented-out headers in output files. These will be ignored by most python I/O functions, like numpy's genfromtxt of pandas' read_tables, and can make output more human-readable.

It could look like this:

Time Eccentricity Semi-major Axis

0.0    0.3                1.0

and so on.