- Added checks in UnitGenerator

- When a unit cannot be built, UnitGenerator does not abort anymore;
error log messages are produced, and it proceeds with the next unit

- Fixed a bug that prevented pipelines in collectagent deployments
- Incremented the tolerance threshold of fuzzy queries to 1h
- Fixed the assignment of MQTT topics to job sensors

- If a subSampling value of 0 or lower is chosen for a sensor, it will
never be pushed out over MQTT/Cassandra and only kept in memory

- Fixed a bug that prevented relaxed mode from working properly for
operators deployed on the collectagent
- Changed the function of the "delay" parameter: it now specifies a
fixed offset applied to the computation interval of each operator,
useful to tune the flow of data in operator pipelines

- Now using references instead of pointers
- Return type is now bool

- If users specify a single timestamp (startTs==endTs) when performing
a sensor query through dcdbquery or libdcdb, a fuzzy sensor query is
performed, and the closest sensor reading to the target timestamp is
returned