Adds UnitTest for partial initialization of DataGroup.
Revert the change to DataGroup::isValid.

The original code asserts that all of the NamedData have the same
validity.
Renames NamedData::isValid to isInitialized.

"Valid" could have different interpretations.  The function tests
whether or not the NamedData object has an IndexDirectory, but not
the existence of anything in that IndexDirectory or the correctness of
any data associated with names/indices.  "isInitialized" is less
likely to be misinterpreted in these ways.

Also, updates documentation and assert messages for clarity.
NamedData asserts on existence of m_directory before accessing it.
Renames DataGroup::isValid to isInitialized.

With the renaming of NamedData::isValid to isInitialized,
DataGroup::isValid should be renamed for consistency.
Also, this avoids the possible confusion users might have about the
meaning of "valid", e.g., whether or not a DataGroup is valid if it
contains bad data.

Also updates documentation and assert messages for clarity.
Removes whitespace at ends of lines in DataGroup.h and NamedData.h