'Add/set the given attr on the asset at the given location. Does not allow overwriting gridFS built in
attrs such as _id, md5, uploadDate, length. Value can be any type which pymongo accepts.
Returns nothing
Raises NotFoundError if no such item exists
Raises AttributeError is attr is one of the build in attrs.
:param asset_key: an AssetKey
:param attr: which attribute to set
:param value: the value to set it to (any type pymongo accepts such as datetime, number, string)'

def set_attr(self, asset_key, attr, value=True):

    self.set_attrs(asset_key, {attr: value})
