No longer require single Closures to be supplied to flow channels

[joshuahansel]

Motivation

The flexibility of the Closures system could be increased by not requiring each Closures object to supply all of the necessary closures. For example, one could do this:

closures = 'my_friction_closures my_heat_transfer_closures'

This allows Closures classes to be more natural, since they don't need to have a combination of types of closure relations.

Design

The closures parameter in FlowChannelBase will now correspond to a vector of Closures names, and it will be optional, defaulting to empty. Then all of the places using it will loop over the provided closures.

Impact

Enhanced flexibility.