doc updates

[harmsm]

Put io.html after basic example, but before full-interface

basic.html

• note that the software automatically infers what genotypes are missing from the input file
• link to io.html for the predictions.csv and scores.csv files

io.html

• update the required inputs (stdeviations and n_replicates are now optional)
• better docs on genotypes.txt (not csv?)
• missing genotypes.txt
• describe what the input columns are; sentence each
• describe the columns of the output files; one sentence each
• big question: what the heck is the 'score' in scores.csv?

[harmsm]

• Update docs to describe what the output graphs mean
• Update the docs to with cross-validation

[harmsm]

All changes made.