Change livetime to time_on in lightcurves?

[cdeil]

We currently have a livetime column in a few of our LC files:

$ ack -l livetime input/data | grep ecsv
input/data/2012/2012ApJ...746..151A/tev-000058-lc.ecsv
input/data/2012/2012ApJ...746..141A/tev-000058-lc.ecsv
input/data/2017/2017ApJ...836..205A/tev-000053-lc-3.ecsv
input/data/2017/2017ApJ...836..205A/tev-000053-lc-2.ecsv
input/data/2017/2017ApJ...836..205A/tev-000053-lc-1.ecsv
input/data/2017/2017MNRAS.471.2117A/tev-000154-lc-1.ecsv
input/data/2017/2017MNRAS.471.2117A/tev-000154-lc-2.ecsv
input/data/2011/2011ApJ...729....2A/tev-000091-lc-2.ecsv
input/data/2016/2016ApJ...831..113A/tev-000168-lc.ecsv
input/data/2016/2016ApJ...831..113A/tev-000169-lc.ecsv

https://github.com/gammapy/gamma-cat/blob/master/input/schemas/lightcurve-format.md

The common meaning is that livetime is dead-time fraction corrected, and is a bit shorter (typically ~ 5%) than the "on" or "observation time. Probably for the lightcurves this is never what is given in the paper, but the "on" or "observation" or "duration" is given?

So I'd like to propose a rename of that column from livetime to time_on, which matches the ONTIME header key of the event list (see http://gamma-astro-data-formats.readthedocs.io/).

@GernotMaier or @wegenmat - OK?