Standardize signatures via dispatch, maybe w/ `trait DistributedFactorGraphs`

[dehann]

ah that .rs signature is so ugly, this the internal function should rather be:

nvasdk::_getId(nvafg, varlbl)

These signatures should instead implement a Trait for more dispatch-like behavior. My sense we need a trait DistributedFactorGraphs and then instead implement the function _getId(impl DistributedFactorGraphs, &str)

Originally posted by @dehann in #154