Muscle3 actor supporting multiple source streams

[maarten-ic]

This PR adds a new muscle3 actor that can connect to multiple Kafka streams and makes it available to the M3 simulation. If the time bases of the streams are not identical, this actor will:

• Follow the time base of the "first" stream (based on the configuration in the kafka_topics setting)
• Skip time points until all streams have data available
• Use "previous time" interpolation for each non-first stream

N.B. This actor requires a development version of muscle3 with support for dynamic O_I and S ports.

TODO:

• Add documentation and usage instructions
• Update MUSCLE3 dependency once the next release is finished