Describe units

[Graeme A Stewart]

We now have a large amount of monitoring in prmon and, obviously this is data that we give as numbers. However, we don't describe properly what the units are, and this is bad.

This should be fixed.

An idea would be that in utils.h, where we setup the column names for monitoring, we'd also record the units. Then we can have an option to describe the units, either as text (for humans) or as a JSON file (for machines). Note we need units for the snapshot/max values and for the average values. They may not be the same.