Add units support in `get_receiver_logs()`

[PietrH]

This idea has been floated around a few times. get_receiver_logs() is the best candidate to try this out at the moment, because there it can easily be made optional, and it has a lot of units that people might want to transform or do math with.

• This can be done based on the column suffix
• I think it would be most efficient to do this before the columns get rectangled into a data.frame, right after/during JSON parsing. Perhaps with a local transform.
• Should be made optional, probably with a add_units boolean argument (discussed with Sanne)
• By default, don't add units, then when requested, check if https://github.com/r-quantities/units is installed
• Some work on this already happened in Add get_receiver_logs() function #254 , but was rolled back into a separate branch not to slow down development on get_receiver_logs()