ENH: more Pythonic datastructure for derived_metrics

[shanedsnyder]

Currently, the derived_metrics value returned by PyDarshan's accumulate_records() routine is just a cdata object matching the C-level struct representation of derived metrics. We are using some less than ideal methods for working with this data requiring us to have knowledge of enum values, etc:

darshan/darshan-util/pydarshan/darshan/lib/accum.py

Line 64 in 05efd1d

# the darshan_file_category enum is not really

We should see if we can make the data more Pythonic so it's more straightforward to use for PyDarshan consumers. Tyler had some quick suggestions elsewhere:

It is true that I don't like how there is a struct than then contains an array which has to be manually indexed based on knowledge of the C code to pull out some counter values. Maybe sketch out a design in another issue if you plan to flatten to a custom class or maybe even dataclass is appropriate... though dunno where it sits on priority side.