[PROPOSAL] When using --use-png, the resolution should be automatically set to be higher

When using `--use-png` as an option to biotea prepare, by default the resulting plots are 12x9 pixels, making them unusable.
Even when setting a higher pixel resultion, the plots are still very small (`png` has the wrong `res` parameter).

Make `png` use inches as a unit of measure, and set a sensible resolution (e.g. 300).
