otp_isochrone output

[jameswoodgeog]

Is your feature request related to a problem? Please describe.
When calculating the isochrones from an sf object, the process produces a new object, and there is now way to tell which line of the output relates to the input file.

Describe the solution you'd like
It would be great to have the option within the parameters to select a column that one wishes to use as a reference between original and output. The default could be the index, but if required, it could be changed.

Describe alternatives you've considered
I have not found an alternative. I am trusting that the output is in the same sequential order as the input (which could prove dangerous for obvious reasons)

Additional context

[mem48]

Do you mean fromID? See https://docs.ropensci.org/opentripplanner/reference/otp_isochrone.html