Implement "Script" Abstraction

[withtwoemms]

As a Script
I want to execute a Procedure sequence
So I can yield results in a single context

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GIVEN that a Script is "evaluated"
WHEN it is
THEN it can be known whether or not it is valid/can be executed

GIVEN that a Script is executed
WHEN it is
THEN it can be done so in a multi-process environment

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releated:

• Ensure objects are pickleable #23
• Actions need to guarantee threadsafe access to resources #28
• Actions should have the option to raise #29

[withtwoemms]

"Script" seems more descriptive than "Plan". Will change issue title/description.