…be picked independently.

The option string --show --intervals is more flexible, --show-intervals is now an alias.

This includes skeleton code written by @martin-cs.

Added hook for running the variable sensitivity

Involved removing messaget inheritance since we require a zero parameter
constructor for ai_baset

Terminate correctly

Made the domain correclty find the fixed point. The merge operation
returns whether the merge actually changed any values by implementing a
operator== for each of the abstract objects.

Further, when merging a map, if a key is absent from one map then we
remove it from the other unless the map is bottom.

Adding handlers for the special types

They are just stubs at the moment, returning top as we need the abstract
objects that represent them.

Updated interface for ai_domain_baset

The base merge should always result in either top or bottom (it is up
for dervied classes to clear the top flag if they are not top).

Correctly use the base class merge when appropriate in merging constant
values.

Refactored the merge maps method to use std::intersection as there was a
bug with empty maps caught by the unit tests.

…esign

Use the CPROVER_assert as behaviour is consistent across platforms.

Removed some unnecessary debug information from the tests

In release builds, the top/bottom status of the enviroment was
undefined. This ensures the enviroment is set to top (i.e. an entry
point).

Previously each abstract object that implemented merge had to correctly
redirect merging top with anything or merging with a bottom.

This is now handled directly in the root merge by checking the three
cases where the base merge must be used.

If this is bottom must still be handled by derived classes as we must
create an abstract object of the same type.

At some point this commit needs backdating as the ai_domain interface
has changed under our feet.