StorageNumpy instantiation limitations

[yolandab]

Current instatiation of a new StorageNumpy supports the following cases:

StorageNumpy(init_array, name) --> Instantiates a persistent StorageNumpy initialized with the values of init_array
StorageNumpy(init_array) --> Instantiates a volatile StorageNuympy initialized with the values of init_array
StorageNumpy() --> Instantiates an empy volatile StorageNumpy

Otherwise, an exception is thrown during the initialization.
For example, executing:
mynp=StorageNumpy(None, "myname")

throws:
IndexError: list index out of range

We should fix this and give an error message to the user saying this instantiation is not supported or implement the same behaviour as for the empty volatile StorageNumpy