Make Gaudi::Functional constructors more paranoid about # of inputs/outputs
Currenly, the constructors of the Gaudi::Functional baseclasses use
std::array<KeyValue,N> as arguments where N is the number of inputs
or outputs specified. However, this implies that one can specify a list
of less than N KeyValues, as std::array will happily
default-initialize any missing items. This will then result in an error
in genConf, as this implies a property with a default-initialized string
(i.e. an empty string) as key.
This is fixed by replacing the use std::array<KeyValue,N> with a tuple
with N KeyValues, eg. for N=2 an std::tuple<KeyValue,KeyValue>.
Warning/Caveat: some (rare) user code may have hardwired the assumption of an std::array,
and thus will have to be migrated. To easy this migration, the version which uses
std::tuple also defines the preprocessor symbol GAUDI_FUNCTIONAL_CONSTRUCTOR_USES_TUPLE,
which can be used to adapt the client code in a backwards compatible manner.