Fix Data.get for unnamed multi-symbol series

[gyx09212214-prog]

Summary

• let Data.get(column=...) select a symbol column when multi-symbol unnamed Series data concatenates to a single DataFrame
• preserve the existing default get() behavior for single-field multi-symbol data
• add a regression test for issue Data.get ignores column argument for multi-symbol data #840

Closes #840.

Validation

• python -m pytest tests\test_data.py::TestData::test_get -q
• python -m pytest tests\test_data.py::TestData::test_concat tests\test_data.py::TestData::test_get -q
• git diff --check