Add option to remove validation checks in `select_filters`

[matildastevenson]

select_filters makes calls to spatial and biocache web services to check that filters are valid. This can slow down requests if a user is redefining filters and making lots of calls in quick succession. May be useful to add an option to ala_config like disable_checks, which can optionally skip validation

[matildastevenson]

Another option is to store fields and layers as data in the package, and refresh only optionally. The problem with this is that a package release will be required to update the data whenever the fields change. Fields and assertions should be relatively stable, but layers change from time to time, and it is repeated layer checking that can cause HTTP 500 errors. Another option, now that assertions are all upper case, is to use this to avoid calls to search_fields