[feature] Split file IO into separate package

[pdimens]

Is your feature request related to a problem and which? Not a problem, but it would be easier to treat file IO as a separate package that is required by and re-exported by PopGen.jl

Benefits

• Simple maintenance because it will only require basic PopData functions
• PopGen.jl codebase will be smaller
• IO development can be independent from other package components
• Contributions will be independent from main PopGen.jl codebase because it will only feature IO-specific things
• cool new logo
• precompile read functions with test data

are there alternatives? Keep the package monolithic as it is now

additional info

[pdimens]

migration has begun on popgenio branch. see PopGenCore.jl.

To do:

• [ ] Register and add PopGenCore as dep
• [ ] reexport PopGenCore
• [ ] remove codebase that was moved into PopGenCore
• [ ] update documentation to reflect changes

[pdimens]

PopData and Genotype/GenoArray have been moved into PopGenCore.jl