Concept for Snapshot Tf Metrics

[fmessmer]

in our scenarios, we want the metrics to check the tf distances at a certain time, e.g. when putting a box to the internal storage
analyzing this distance as the minimum over the whole recording period is unreliable as we might move closer to that goal pose during some other phases of the scenario

the minimum approach only works for groundtruth.data: 0.0 - but when there is no tf frame at the goal pose this is not the case
#179 might also be a solution for the problem

[floweisshardt]

Snapshot metrics concept is already supported with mode: 0 (=SNAP) (see https://github.com/floweisshardt/atf/blob/master/atf_msgs/msg/MetricResult.msg#L1). The snapshot is triggered when a testblock is stopped.

So for the above scenario you would need to define a testblock which you call atf.stop() at the point in time when you are interessted in the tf distance.