Field filtering: if we remove all fields, verify fails

[mclarsen]

1. Verify: should a mesh be required to have a field?
2. If 1, then we should cull all topologies that have no remaining fields.

This happens if we have two topologies and only plot a field from one of them.

[mclarsen]

@cyrush What do you think about 1?

[mclarsen]

Implementing #567 would resolve this.

[cyrush]

fields are optional -- folks may just want a mesh plot.