Re-organize code: Break up resampling.jl

[ablaom]

We break up the very long src/resampling.jl file into separate files:

resampling
├── error_messages.jl
├── evaluate.jl
├── evaluation_results.jl
├── logging.jl
├── resampler.jl
├── strategies.jl
└── train_test_pairs.jl

This PR contains no code changes of substance.