Avoid silent errors accessing Fieldset fields by ambiguous names

[wdeconinck]

This should address #194

An exception is thrown when accessing a field by name in the fieldset, when the name is ambiguous. A unit-test is created that demonstrates the exception.

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[DJDavies2]

I seem to be getting a number of failures in downstream codes with this like so:

Exception: FieldSet: cannot get field with ambiguous name height_levels". 2 fields are registered with same name. Access field by index or iterator instead. (/home/h01/david.davies/cylc-run/atlas-210/share/mo-bundle/atlas/src/atlas/field/FieldSet.cc +120 field) FieldSet: cannot get field with ambiguous name height_levels". 2 fields are registered with same name. Access field by index or iterator instead. caught in (/home/h01/david.davies/cylc-run/atlas-210/share/mo-bundle/oops/src/oops/runs/Run.cc +197 execute) Exception: oops::ErrorCovarianceToolbox terminating... Exception stack: FieldSet: cannot get field with ambiguous name height_levels". 2 fields are registered with same name. Access field by index or iterator instead.

[wdeconinck]

Thanks @DJDavies2 , this are exactly the silent errors mentioned in #194 (notifying @fmahebert @svahl991)

[wdeconinck]

@svahl991 I updated the testing :)

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[fmahebert]

@wdeconinck Thanks for this!