Move SEER data into separate package

[gadenbuie]

Current fcds includes SEER population data for US, Florida Counties and the 2000 standard population, in addition to the SEER ICD-O-3 validation list.

I've filtered out population data for years that don't match the mid year of the current FCDS release, but this means that the built-in data only works with the latest FCDS release.

I see some benefits to creating a data package with cleaned SEER data, i.e. the relevant population data, ICD codes, etc, using fcds as a starting point for what to include.