GetObservableValue input obs name without process name prefix

[nkx111]

This closes issue rest-for-physics/detectorlib#47

By making TRestAnalysisTree::GetObservableValue<>() searching for observables, one can directly input obs names without process name prefix. It also prints message if one input observable names wrong.

Previously:

root [1] ana_tree0->GetObservableValue<double>("sAna_ThresholdIntegral")
(double) 161060.00

Now:

root [0] ana_tree0->GetObservableValue<double>("ThresholdIntegral")
TRestAnalysisTree::GetObservableID(): Find observable sAna_ThresholdIntegral for obs name: ThresholdIntegral
(double) 161060.00

root [2] ana_tree0->GetObservableValue<double>("ThresholdIntegrawwl")
-- Error : TRestAnalysisTree::GetObservableID(): No Matching observable found
-- Error : did you mean "sAna_ThresholdIntegral" ?
(double) nan

When writing processes, if one needs to access observable values, no more hardcoded process names shall be added.

[jgalan]

What happens if there are several analysis processes that generated the same observable name with different process name, it takes the first one found?

[nkx111]

What happens if there are several analysis processes that generated the same observable name with different process name, it takes the first one found?

It will prompt warning and return the first one matched

https://github.com/rest-for-physics/framework/pull/290/files#diff-dac4ddbd262635783e441e810b53ad77e58453a25315d096cea9f8e02c4bf238R211

[jgalan]

Shall we merge this PR?