Add support for phenology observations in the Netherlands

[amotl]

Dear @TheDiren, @pvgennip, and @beepnl,

at ^1, we are presenting a new subsystem for the phenodata program to export data into SQLite databases for easy consumption in downstream applications. I think it could be sweet to follow up on this by looking at whether the Dutch government also provides unrestricted access to phenology observation data. Maybe you can conduct some research, and share back corresponding pointers with me? Thank you so much!

With kind regards, Andreas.