Human readable output for data read command

Default output from the _data read_ command is now human readable, with a `--raw` option to return the actual values (as rendered by their Show instances). The output formats dates in ISO 8601 and simple payloads as either integers or floats, according to their magnitude.

AfC
2^51 was somewhat arbitrary. You're right, it was significand - 1. Seemed safe to assume anything larger than that was in fact an encoded float; 2^32 is too small (we have counters bigger than that) but nothing has gone past 10^18 so there seemed plenty of headroom.

In any event, this is just a best-effort human readable display. If you actually need the value you get it out raw (aka Show), or as actualy bytes via the proper ReadRequest pathway.

I'll add a comment along these lines.

AfC