override global info instead of replace on __init__

[Teque5]

Override not overwrite global information when creating new SigMFFile.

Closes #76.

Should this be the behavior for set_collection_info too?

# currently
def set_collection_info(self, new_collection):
    """
    Overwrite the collection info with a new dictionary.
    """
    self._metadata[self.COLLECTION_KEY] = new_collection.copy()