JSON schema corrections

Fixes a few things: 

1. Prevent schema from accepting null values for `keyword` which should always be required. Allowing it to be null was an oversight in the update on #269 
2. Make sure each entry in `keyword` is at least one character long (prevents empty strings)
3. If `distribution` is used then ensure that both `accessURL` and `format` are included 
4. Defines regular expression pattern for `PrimaryITInvestmentUII` as mentioned in #283 and #282
5. Adds `accessURL` and `format` as possible fields outside of `distribution` (but can be null)
6. Made it so a `keyword` doesn't have to be unique as discussed in #288