Fix --clean flag in federated generator

[axmmisaka]

This hopefully completes the task mentioned in #1221.

This PR did......

1. Added FedGenerator::cleanIfNeeded() just like other generators; 86681d6#diff-e0f660764015b35c4b0bf3bb21720b1f7be727746c3790043b30139de8f90671R174
2. Override FedFileConfig::doClean(), 86681d6#diff-aef6a300a7e1bac3cddb7efb7226ba4b7e59333e269617fd75d591baaad0a5e3R85

So in total, it will clean ./src-gen (which will not exist if federated), ./bin (appears to exist regardless of federated) and ./fed-gen/{name} (unlike in non-federated context, removing the whole directory makes sense as the whole directory is generated).

@lhstrh mentioned to me that one more thing to do is to tell each federate to do --clean; I'm unsure if that is been done or not now as during federate generation context which possibly contains clean flag will be passed in:

lingua-franca/org.lflang/src/org/lflang/federated/generator/FedGenerator.java

Line 160 in 86681d6

context, federate, federates.size()

Marking this WIP until I confirm with people the expected behaviour of --clean under federated: would cleaning these 3 places be enough

[lhstrh]

Minor change requested. Otherwise looks good!

[petervdonovan]

The requested change has been made, and I am in such a hurry to get fed-gen merged that I will take the liberty of merging this branch into it. If any lingering issues remain, we can deal with them later, maybe in the fed-gen branch.

[lhstrh]

I think it's a good decision. Thanks for being so proactive, @petervdonovan!