network option should not require number of nodes in CLI

[pmendes]

When specifying the connections with a network file the user should no longer need to include the number of units in the command line; this is because the network file implicitly specifies how many units will be needed. This would minimize situations where the network file has, say, 10 units, but the user specifies 11 units in the command line. Presently the code checks for this discrepancy and aborts with an error message.

For this to work we need to:

1. change the logic of the command line arguments such that no number is needed when --network is present
2. explicitly update the number of units in the code reading the network file
3. maybe add a more robust check for the units to be named as sequential numbers