Implement SpurResource class

[peterlai1]

As per #58

Additional changes:

• Added a custom SpurRequest class as well that inherits SimPy's Request class; this custom class also takes in the agent making the request, so that in the _do_put() method of SpurResource, the associated component's check_usage_eligibility() method could be called with the specific agent.
• The accept_agent() method is now called directly within the _do_put() method, such that if the request is successful, the agent is added into both the resource's list of users and also the component's own data structures as well; this synchronized update eliminates event ordering issues arising from concurrency.
• Related to the above, release_agent() is now invoked in the main tour traversal loop of the Train class, instead of being part of the transfer_to() method.

[wklumpen]

If you want to commit/push the two small changes (method name and type hinting) we can wrap this one up.

[peterlai1]

If you want to commit/push the two small changes (method name and type hinting) we can wrap this one up.

Renamed to can_accept_agent()