Multiple time slices per file

[dabail10]

Something I keep forgetting about is adding the capability to CICE to have more than one time slice of history output per file. The operation of opening a new file each time can be costly, particularly at high resolutions and with daily or higher frequency output. This involves some book-keeping in terms of the file names.

[apcraig]

Also, as noted in #624, "At some point, I would like hist_avg to be an array of size nstreams. Then we can control the instantaneous versus average on a stream by stream basis."

[phil-blain]

Also, as noted in #624, "At some point, I would like hist_avg to be an array of size nstreams. Then we can control the instantaneous versus average on a stream by stream basis."

This bit was done in #827.