when fitted model params are not finite, summary stats blow up

script `runPerformanceTests.py` parses output of stansummary (I think),
but param values can look like this:
```
tau[2]          1.0e+201      nan      inf  2.8e+199  2.3e+200  4.5e+201      nan      nan      nan
```
and the functions which parse this summary (`csv_summary` and/or `parse_summary`) blow up.
use `math.isfinite` to check values before trying to compute average?
