Improve TM based on user feedback

[holgerbrandl]

In particular

• better document input data prep and layout
• rework input data validation to give more meaningful feedback (e.g. pull out original.png generation into upstream tool).
• polish mac setup and reference it better in docs

The mac setup may also require some code changes if the underlaying R packages have changed (like ggplot2 did so a lot more recently).

@etournay Would you mind if we (me+@jandoG) slightly evolve TM based on the internal feedback from Eaton Lab or do you have a different roadmap in mind already? It seems that TM is still very popular there, so we'd like soften some of its sharp edges and push it gently forward where needed.

[etournay]

Hi Holger,
For some reason I didn't get informed of this issue (due to holiday time most likely).
Yes, go ahead for modifying TM.
I would suggest that you create a new branch.
Best,
Raphael