[QC-1020] Fall back to default values if beamtype or runtype cannot b…

[Barthelemy]

…e found.

[Barthelemy]

@aferrero2707 This is what you asked for the CustomParameters: if a value is not defined for the requested beam/run type, we use the one defined for "default" (if any).

To me it seems a bit confusing but I see the benefit as well.

[aferrero2707]

@Barthelemy is there a specific reason why there is no implementation of this version of at()?

std::string CustomParameters::at(const std::string& key, const Activity& activity)

[Barthelemy]

1. I added the missing version of at
2. loop: yes
3. doc: yes

Thanks !