Allow unknown keywords to be read

[tim-vd-aardweg]

Is your feature request related to a problem? Please describe.
In #571 we added a lot of new keywords. However, we only added keywords that are the manual. There are many more keywords being printed to the .dia file. In most cases, these are research keywords. We want a way to support the reading (and writing) of these keywords as well, without having to add them to the source code.

Describe the solution you'd like
We should probably set Extra.Allow to support this. We have to look into the best place to add this and what the consequences of this change are.

[tim-vd-aardweg]

Discussed this issue with @arthurvd @rhutten @veenstrajelmer @MRVermeulenDeltares . We will not be supporting the reading and writing of unknown keywords after all. Mainly because there should be no unknown keywords at all. We will aim to add all currently 'unknown' keywords to HYDROLIB-core. Furthermore, we will make a tutorial for how the users can add keywords to models themselves. See #637.

I will therefore be closing this issue.