feat: Enter accrual periodicity via drop down

[hbruch]

According to dataset-specification, values for accrual periodicity should conform to ISO 8601, which provides much flexibility but puts a burden on the user to figure out the correct syntax.

I suggest to follow the Dublin Core Metadata Initiative and use a fixed catalog that can be proposed as drop down for entry: http://purl.org/cld/terms/Frequency.

However, if no drop down is provided, an example should be provided in the text field. The tooltip should cite the expected format.