DL3 stage

[morcuended]

The preliminary version of the DL3 stage recently implemented in #81 is very simplistic. We would need to deal with different-type IRFs.

For now, we could produce DL3 files without adding IRFs nor creating the DL3 index files. To do that we have to run the DL3 Tool, without including the IRF file, and it will just save the DL3 file. At a later stage, we could add to it the 'right' IRFs, and run the index file for the selected DL3 files.

In this sense, there is no need for storing the DL3 files in separated directories on a source basis. We might just put them all together on a nightly basis as it is done with previous data levels.

I leave this issue open to have it in mind for future implementations.