Certain tsv columns are specified in the spec but their type information can be overridden in sidecars using the header name as a key and specifying a "Levels" or "Units" field. The schema validator should conditionally do type checking on a columns values using the schema default definition if the column field is required. If a field is recommended the type check should be a warning, and if a user overrides a recommended column definition the sidecars definition should be used. Additional columns should be type checked soley on the sidecar definition producing warnings. Finally if a user attempts to redefine a required column using "Levels" or "Units" a warning should be thrown.

@effigies does this lineup with what we discussed? I forget if some recommended fields should never be able to have their types overwritten.