Trim values in property class

[JensWehner]

Should we trim values, i.e should

<tag>A </tag> and
<tag>A
</tag>

be equivalent to <tag>A</tag>
We do that to string values already but it could violate the XML specs.

[junghans]

We do that in csg_inverse, too (https://github.com/votca/csg/blob/master/share/scripts/inverse/functions_common.sh#L249), so fine with me as long as we only trim space/tab as the beginning and end.

Kind of what trim_all() does: https://github.com/votca/csg/blob/master/share/scripts/inverse/functions_common.sh#L362-L365

[JensWehner]

spce tab and linebreak from the end and beginning of the value?

[junghans]

Yes

[JensWehner]

should be fixed by #365