Make it possible to use saved values with ElfiStore

[jlintusaari]

Elfi nodes should be able to use previously stored data. Being able to safely produce additional data to the stored data requires that we know at least the master seed and sub stream index, so that we don't repeat already computed values that are in the store.

Some tentative ideas

Add to ElfiStore an optinal mask for stored values. When using numpy arrays, use the numpy.ma

[vuolleko]

Supported as of 0.5