Implement impact level as a statement about a system of type "fisma_impact_level"
with imoact level in body. This continues idea that statements are
factual observations about a system.  Statement must be associated
with system.root_element and be consumed by root_element.

Enumarate different type of statements

… methods

Set system fisma_impact_level root_element statement as part of
question action to set baseline. Since baselines could have more
names than just "Low", "Moderate", and "High", we only set
fisma_impact_level if baseline is one of those three.
This might need to improve in the future and make setting fisma_impact_level
a separate question action from setting baselines.

Created set/get methods for fisma_impact_level in System model
to make sure only one statement for the system with statement_type of
fisma_impact_level existed.

Also fixed POAM stat counting to only count POAMs associated with
current system. Existing POAM count was not filtering on current system.

all caps no brakes