Include km3net-dataformat as an external

Currently, the use of km3net-dataformat is an option in KM3BUU, and therefore it exists the possibility to run it without producing the results in the km3net-dataformat.

In order for it to comply with the envisioned strategy that all the km3net software should produce the results formatted accorting to the km3net-dataformat, I proposed that the km3net-dataformat repository is added as an external to this repository. The resulting singularity container will allow the user to obtain the results directly in km3net-dataformat without the need of any conversion.

@tgal, we decided to discuss this in an issue to get your feedback. What do you think?