Only fail validation of cash letter control for settlementDate field if it is not a return type

[SaraAdineh]

Here is the original discussion on this matter.

According to different specs, the SettlementDate is a conditional field. Currently, the parser requires this field to exist. Having this field present for Return bundles does not make sense hence why they are defined to be conditional.

There were two approaches to take:

1. To remove this check completely.
2. To not have it required for Return documents which are indicated by CollectionTypeIndicator.

I went with the second approach.

[adamdecaf]

I like the idea of passing contextual knowledge over to Validate. It looks like we'll need to do more of this to support all X9 specs.