New aggregator plugin "basic_aggregator"

Hello,

basic_aggregator (src/basic_aggregator.c) is a new aggregator.
In the config file, you specify the aggregator type, what to aggregate (types or type instances or all instances of types) and how (currently, sum and average).

Interesting feature : if you ask for it (in the config file), this aggregator can find by itself all the instances of the type you want.

Regards,
Yves