A read-only object indicating whether or not this history
bucket contains valid data. Valid bucket is reported as true(1)
and invalid bucket as false(2).
If this history bucket is invalid the BTU-C MUST NOT produce
notifications based upon the value of the counters in this
bucket.
Note that an implementation may decide not to store invalid
history buckets in its data base. In such case this object is
not required as only valid history buckets are available while
invalid history buckets are simply not in the data base.
This object maps to the TR-159 attribute
aGroupPerf1DayIntervalValid.