`is` and `not` operators

The ADC API specifies the is and not operators as being (very confusing) shorthands for is missing and is not missing, see the operators listed under "Filters Query Parameter":

https://docs.airr-community.org/en/latest/api/adc_api.html#request-parameters

Therefore, they are also operating on 'field' and lack a 'value'.

Currently the shorthands are implemented them as is_ and not_ functions, which is not the required behavior. Furthermore, the long versions of the operators are not implemented at all