fix interface to libsml

- Ignore string entries and not valid entries like error codes (81 81 ...). Refer to issue #29 this removes the "unknown type error from libxml". Unknown type have actually been octet strings.
- Report correct number of readings. There was always a +1 wrongly reported and the octet string ones that get ignored now. Discussed as part of #63.
- Added basic unit test for MeterSML using the example data provided by devZer0 from #63.
- Includes unit test for Obis and fix for Obis (isxdigit -> isdigit and ...) (replaces/includes #59)

With those changes now correctly just 3 readings get extracted from the example data.
Needs good testing with SML based meters. I will support debugging any occurring problems.