checkForUnusedOptions should gather across MPI ranks

[bendudson]

The checkForUnusedOptions throws an exception when options are unused. This incorrectly throws when expressions are used to set boundary conditions in multi-processor runs e.g. bndry_core = dirichlet(core_ne) the core_ne option is used on processors that have a core boundary, but not on other processors.

PR #3365 was an attempt to work around this by adding zero-width boundaries to all processors, but that doesn't work and would be fragile.

Since checkForUnusedOptions is not performance-critical, we can afford to do some kind of reduction over all processors.

[ZedThree]

I feel like this has come up before and we had a fix already, but for the life of me I can't remember what it is.

[bendudson]

Hey @ZedThree ! There is setConditionallyUsed but that doesn't parse the option so

[var]
bndry_core = dirichlet(1.0)

will work if bndry_core is set as conditionally used, but

some_value = 1.0
[var]
bndry_core = dirichlet(some_value)

The some_value option won't be marked used. One possibility would be to modify setConditionallyUsed so it recursively marks expressions as used. I think a better solution is to do a reduction over all processors (as @dschwoerer suggested).