Make encode-marc21 validation optional or optionally less strict

[TobiasNx]

Currently the marc encoder only serializes to marc21 if a lot of validations are true.

Hbz/Verbundgruppe has a use case where for the it is more important to change the form from marcxml to marc21 than having valid marc21.

The transformation breaks if the data is not valid at the moment.

I therefore suggest to make the validation of the leader and controllfields optional.

[TobiasNx]

Also the marcxml schema is less strict than the marc21 specification from LOC:

https://www.loc.gov/standards/marcxml/schema/MARC21slim.xsd

Leader has to have 24 characters but the regex is the following: <xsd:pattern value="[\d ]{5}[\dA-Za-z ]{1}[\dA-Za-z]{1}[\dA-Za-z ]{3}(2| )(2| )[\d ]{5}[\dA-Za-z ]{3}(4500| )"/>