Ammeters don't update after deletions

[phet-steele]

Very similar to #407, but deals with components being deleted instead of moved. Right off the bat...the voltmeter seems to handle things well (or at least it's harder to make break), so ignore it in this issue.

1. Setup a circuit like this:

2. Delete the wire the ammeter is measuring. The ammeter will still show current. The in-line one will be correct.

3. Now setup the same thing, except make the battery 0 V.

4. Delete the wire the ammeter is measuring. Both ammeters will still show current. So when the in-line ammeter measures 0.00 A before the deletion, it breaks?

To be clear, the problem is that ammeters will still measure current when there should be none. Moving a circuit element will fix the readouts.

Seen on macOS 10.12.6 Chrome. For phetsims/qa#51.
URL: https://www.colorado.edu/physics/phet/dev/html/circuit-construction-kit-dc/1.0.0-dev.166/circuit-construction-kit-dc_en.html
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[ariel-phet]

@samreid this seems like a more legit bug. Since moving circuit elements fixes it, I could live with it for 1.0, but would prefer not to. Worth investigating a fix.

[samreid]

Proposed fix is above, @phet-steele can you please investigate?

[phet-steele]

@samreid steps 1 & 2 are fixed, but 3 & 4 haven't change for the SERIES ammeter (the contact ammeter is fine). Starting at 0.00 A, the series ammeter doesn't update after a delete:

[samreid]

Many thanks @phet-steele, proposed fix committed.

[phet-steele]

Works great, thanks @samreid!