Add support to composite partition keys in StorageDict

[yolandab]

Extend the data models that can be specified with Hecuba to support composite partition keys. Until now the attributes that compound a primary keys are specified between <>: the first attribute is considered the partition key and the rest are clustering keys:

class mydict(StorageDict):
'''
@TypeSpec <<attr1,attr2,attr3>, attr4, attr5> # attr1 is the partition key, and attr2 and attr3 are clustering keys
'''

From now on, several attributes can compound the partition key if they are separated by ":":

class mydict(StorageDict):
'''
@TypeSpec <<<attr1,attr2>,attr3,attr4>, attr5, attr6> # attr1 and attr2 are the composite partition key, and attr3 and attr4 are clustering keys
'''