Add groups for producer and consumer nodes

[peetonn]

Short
Currently, there might be only one type of producers and one type of consumers.
It makes sense to support multiple types (or logical groups), though.

Detailed
For example, in ndnping example all producers, regardless of how many could be defined in the .dot file will run same command, defined by ENTRYPOINT in producer.env file - which means all of these producers will register same prefixes. It is not an erroneous behavior, but it is a limiting one - a user might want to start several producers with different prefixes each and then setup consumers groups for each producer.

How
Grouping might be implemented by:

• first, defining groups in .dot file
  • for example, producer/consumer labels can now define group in parentheses, like P1 (3): /ndndn/test or C17 (2) <- P1
• second, each producer/consumer group needs to be provided with a separate .env file
  • it means that ndndn generate arguments -p and -c should take variable number of values:

    ndndn generate -p producer1.env producer2.env producer3.env \
                   -c consumer1.env consumer2.env consumer3.env ...