Specify default value for dmax, method to safely parse dmax and default to 0

Change the XML Doctype in configuration files `jmxetric.xml` and `jmxetric_test.xml` such that the `dmax` attribute has a default value of `"0"` instead of `""` previously.

Add a new method `parseDMax` to safely parse the `dmax` values that were read in. It will call `Integer.parseInt` on the value, and defaults to `0` if parse fails.

Fixes #22