fix setting of dataframe indexes for tsv generation in templater

[JuliaSprenger]

fixes #108

[coveralls]

Pull Request Test Coverage Report for Build 2496168222

• 8 of 8 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 72.939%

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Totals
Change from base Build 2495145429:	0.1%
Covered Lines:	938
Relevant Lines:	1286

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💛 - Coveralls