Implement an environment for SPN

[fsaad]

Consider the following SPML program:

X ~ Normal(0, 1);
if X > 0:
    Z := X**3
else:
    Z := 1 + X

probability(Z > 0);

Here, each sub-SPN in the overall mixture induced by if needs to be able to map Z differently.

[fsaad]

It is not likely to be useful implementing the environment directly in the interpreter, since we would need have to store nested environments induced by nested SPNs. The cleaner solution is to have the SPN manage it's own environment.

[fsaad]

Naturally, SPML should disallow redefining variables.

[fsaad]

The new SPN method transform will record the transformed variables in the environment.

[fsaad]

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