Loosen error tolerance in test.
Loosen error tolerance in test.

The PROPKA31 tests fail on Windows 10 Ubuntu 18.04 Windows Subsystem
Linux without any changes.  The relative error is less than 1e-3.  This
commit chnages the behavior to a test of absolute agreement to a test of
relative error less than 1e-3.

jensengroup/propka,jensengroup/propka-3.1