filters.selector was kind of useful

The old `filters.selector` could do some things that are not possible under the new regime. First, because of the way schemas worked, you could mark dimensions as ignored. Second, you could create new dimensions that other filters could do stuff with, including write them out.

Consider the following scenarios:

1. I want a writer that understands a dimension type, say ScanAngleRank, to zero it out when the writer writes it to disk. 

2. I want to read data that has no RGB info and write it into a format that contains slots for RGB. This is pretty common when taking LAS data (many sub-formats of LAS do not have color slots) and write it into pgpointcloud. I could add magic to the pgpointcloud writer to write color slots, but the `drivers.sqlite.writer` or `drivers.oci.writer` have the same problem. 

How can we get these behaviors under the new regime?