findWithDefault

[baconpaul]

One thing I find myself doing a lot is

 if (bitmap_file_registry.find(filename) == bitmap_file_registry.end())
        return nullptr;
    return bitmap_file_registry.at(filename);

and what I really wanted to add is (basically)

T& findWithDefault(const C& container, const T& defValue)

so I could write that as

return sst::cpputils::findWithDefault(bitmap_file_registry, filename, nullptr)

just writing that here so I remember :)