(Re-)Build of executables

The davai-build command does 2 sequential things:

Fetch sources interactively.
Build in each flavour, submitting one batch job building sequentially for each compilation flavour (DP, SP, compiler...).

Tasks:

gitref2pack : fetch/pull the sources from the requested Git reference and set one or several incremental gmkpack's pack(s) – depending on compilation_flavours as set in config. The packs are then populated with the set of modifications, from the latest official tag to the contents of your branch (including non-commited modifications).
pack2bin : compile sources and link necessary executables (i.e. those used in the tests), for each pack flavour.

In case the compilation fails, or if you need to (re-)modify the sources for any reason (e.g. fix an issue):

implement corrections in the branch (commited or not)
re-run the build:
```
davai-build -e
```
(option -e or --preexisting_pack assumes the pack already preexists; this is a protection against accidental overwrite of an existing pack. The option can also be passed to davai-run_xp)
and then if build successful davai-run_tests

Not implemented yet.