Consider the normalised encoding of date ranges in @notBefore and @notAfter when using ISO 8601 encoding

Creator of issue

Text: Following up on https://github.com/SAA-SDT/eas-schematrons/issues/7 and the realisation that the schematron currently does not allow date ranges in @notAfter and @notBefore when used with ISO 8601 encoding, the intended usage of these attributes - especially in comparison to @standardDate which allows for date ranges compliant with ISO 8601 - should be confirmed.
It seems to make sense, initially, to restrict these two attributes to giving one earliest respectively latest date as a single date also in normalised form, but if someone wanted to e.g. say "not before the 18th century" in a standardised way according to ISO 8601, the value "1701/1800" would not be valid in @notBefore with the current setting. This case could be made for the description of archival materials in the same way as for the description of agents, e.g. regarding uncertain dates of birth or death, for which one might want to give an indication of e.g. "not before the 1720s" or "not after the 1750s".
It should be noted, that this is specific to the case of having ISO 8601 defined as the date encoding standard applied for @notBefore and @notAfter. If @dateEncoding is used with the value "otherDateEncoding", the option to encode date ranges in these two elements already exists depending on the encoding rules applied.