Reconsider comp flattening behavior for unimplemented packages

[luciansmith]

There are a few options in the comp flattening converter that didn't interact with each other intuitively for me for unimplemented packages, namely:

stripUnflattenablePackages (true/false) abortIfUnflattenable (none/requiredOnly/all)

I believe there is a difference in behavior for required packages that are unflattenable vs. required packages that libsbml doesn't know about at all. It seems that 'stripUnflattenablePackages' only strips the former, not the latter. Need to do some more testing to confirm this, but I think that's the general gist.