Write function to produce the .json file for respilens

[Mmietchen]

We want a function to_respilens(model_data, target_data) to produce the .json file required by myrespilens.

model_data : that's the output of get_fcast see: x$hubcast$model_out_tbl in the draft example.
target_data : that's just the df from get_data

[CyGei]

See latest commit to write-to_respilens branch. It could still use more simplification/less verbose if someone has time (@bestill3 ?)

[bestill3]

I provide a few code review comments for write-to_respilens, we can discuss from there.

[emprzy]

resolved with #41