add sharding before writing

We want to shard all gridfs chunks collections between the different servers, this implies that before being filled with data, sharding needs to be activated for these collections. Some sample code implementing this is in readHeader.py, lines 106 onwards, @guenter knows how it works.

This functionality could be implemented as a create_gridfs(collection_name, activate_sharding=True) method in SDCMongoClient which is then called in the __init__ functions of inheriting classes