export subset of strands

[dave-doty]

In the export methods (eg, write_idt_plate_excel_file), allow only a subset of strands to be exported.

[dave-doty]

TODO: make an equivalent web issue

[dave-doty]

While we're at it, let's also allow a strand to have vendor_fields specify a plate name but no well: if so when exporting, we use the plate name but auto-calculate the well position.