Rename (or alias) `RelayMatrixVirtualMux` as `BreakBeforeMakeMux`

[clint-lawrence]

The point of that mux subclass is to explicitly ensure break before make switching. The name is really just a side effect of what hardware we were using when the motivation came about.

It is probably work creating a MakeBeforeBreakMux also at the same time. I know such behaviour has been manually created in at least one of our test scripts.