Consider using lxml for RDF/XML parsing

[Haigutus]

Is your feature request related to a problem? Please describe.
Speedup of CIM/XML import. Relevant for EU IGM/CGM process, with tight deadlines.

Describe the solution you'd like
Instead of builtin xml parser of python use lxml

Current state:

VeraGrid/src/VeraGridEngine/IO/cim/cgmes/cgmes_data_parser.py

Line 8 in 56e589d

import xml.etree.ElementTree as ET

Possibly faster parser setup:
https://github.com/Haigutus/triplets/blob/cbcc1bdc143b7199416f456c007e56f4619117a7/triplets/rdf_parser.py#L228

[SanPen]

I've been doing some improvements and lxml does not seem to make a big difference. Changing to streaming seems to be better.
The overall performance is much better not because of some other design modifications.