Added support for properties as meta-data

Jan Pöppel requested to merge jpoeppel:properties into master

Created by: jpoeppel

XBIF supports the PROPERTY tag for meta-data on the network, the variable, and the definition level. These changes add an optional flag to the io-methods parse/write called ignoreProperties, which will ignore those fields by default (old behaviour) but which can be set to False, which will result in the properties for the network and for the variables to be stored in a new "meta" attribute. When writing these fields will again be ignored unless otherwise specified.

Since the PROPERTY tag has no structure, we are only storing the plain strings in a list in the meta attributes.

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