Change default preservation on download web interface

[pnovack-gottshall]

Request: Change the default on https://paleobiodb.org/classic/displayDownloadGenerator for "Select by taxonomy > Preservation" from "all taxa" to "regular taxa only" (i.e., "pres=regular").

Rationale: Most users intend to download body fossil records. The current default also includes trace fossils and form taxa, which can cause confusion for less experienced users. (This request was supported by members of the ExCom at our last meeting. I recall Patricia Holroyd first suggested the change.)

[pnovack-gottshall]

While at it, would be also useful to have an option for including BOTH "regular and form taxa" (i.e., "pres=regular,form")