Improve compatibility with PCN format requirements

[dirk-qualisys]

This is a successor of issue #124, to check and improve implementation of PCN related information. At least, the reason should be added why a pH value is not relevant.

[dirk-qualisys]

Will be added to the general PhValue type in spite of PCN being a European requirement, because

1. pH value is a property, the reason for its absence is interesting in all legislations and part of Iuclid which is set up for worldwide applicability.
2. The information is necessary e.g. when importing a private label mixture from outside the EU (that is, the supplier does not devliver documents according to EU requirements but the recipient needs to notify according to PCN).