Add symbol and symbol collection to exports in __init__.py

[AlecGoncharow]

https://github.com/BridgesUNCC/bridges-python/blob/master/bridges/__init__.py

[esaule]

@AlecGoncharow I don't understand what that means. Also, is it critical as in need to be fixed right now?

[AlecGoncharow]

Basically as it stands all other data structures are imported by default with the import bridges

meaning:

import bridges

my_color = Color('red')
my_array = Array()
"""
...etc are bridges data structures
"""

This is not the case with the symbol collection stuff. I am not sure why, but it would make sense to be consistent.