Support for string fields in statistics

[jjwilke]

We currently only support output of integer/float types. There are certain scenarios where it would be useful to output strings (extra metadata fields). This is already done to a certain extent by dumping extra metadata like "StatisticSubId" This should really be unified by having statistics register these as regular fields in the base statistic, rather than replicating the logic to output StatisticSubId and related metatdata for every output. This should be easy for CSV, JSON, TXT, and console.

This is particularly problematic for HDF5 which doesn't output these at all. HDF5 has predefined C_S1 type - so this should be straightforward.