- dont cache HTTPS response in file
 - catch exception if value could not be read

Instead of using 2 specific IDs, a generic path to the value in the XML-file has to be given now.
This enables the plugin to also be used for other XML use cases apart from specific PDU-case.

2 Bugfixes:
-correct initialization of PDUSensors with new()
-cooldown value for XML-file refresh is now correctly converted from MS to NS

Remaining TODOs:
network logic to obtain XML-file
use appropriate lock when sensor accesses PDUUnit
make reading XML more generic?
Test it!