New issue 627: field_list on geometry needs to have the field types as well as field names https://bitbucket.org/yt_analysis/yt/issue/627/field_list-on-geometry-needs-... Matthew Turk: Currently, much of the field detection code is complicated by the fact that we have fields like `"Density"` inside the `field_list` as opposed to `("gas", "Density")`. We should resolve with two improvements: * `field_list` should be a subclass that checks against the various items and field types * `field_list` should also only have the proper tuples Once this has been done, much of the code in the field detection and dependency verification can be eliminated. Responsible: MatthewTurk