v0.2.0

• Added new gnmi_subscribe() task. It supports all telemery subscription modes; however, from the Nornir perspective, the most benefitial is the once mode. In this case, the task will return a list containing output of all requested data. Such an approach is recommended by some vendors (e.g., Nokia) to collect huge data sets, which are not fitting into a single Get() RPC implemented in nornir_pygnmi as gnmi_get() task.