Fix DataFrame indexing error in RTS-GMLC parser

[darrylmelander]

Fixes # .

The RTS-GMLC parser references a particular DataFrame cell by row key where it should reference by row position. The test didn't catch this because the first row of the data frame has a key of 0 in the RTS-GMLC data we test against. That isn't guaranteed to the case in other data sets.

Summary/Motivation:

Changes proposed in this PR:

• Fix indexing

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