Calculate daily means from 5-minute interval data

Regardless of whether you use the lower-level split function, or the
higher-level aggregate function, or the tidyverse group_by function, the
key is learning how to create the column that is the same for all records
corresponding to the time interval of interest.

Jeff,

I tried responding to only you but my message bounced:

<jdnewmil at dcn.davis.ca.us>: host
     d9300a.ess.barracudanetworks.com[209.222.82.252] said: 550 permanent
     failure for one or more recipients (jdnewmil at dcn.davis.ca.us:blocked) (in
     reply to end of DATA command)

My response was not pertininet to the entire list, IMO, so I sent it to your
address.

Rich