Comb-spec-searcher

Added
- added an optional `fixed_values` parameter to the `DisjointUnion` constructor,
that allows you to set a value that a child's parameter must take.
- add the abstract method `can_be_equivalent` to `AbstractStrategy`.

Changed
- removed the method `is_equivalence` from `Constructor`. You should instead
use the `is_equivalence` method on the `Rule`.
- the `CartesianProduct` now considers compositions of all parameters and
not just `n`.
- the `RelianceProfile` type changed to work multiple parameters. It is now a
dictionary pointing from parameters to the values.

Fixed
- ignore rules where the left and non-empty right hand sides are the same

Added
- Support for maple equations in multiple variables
- an option on `auto_search` to not expand verified classes
- a `LimitedStrategyRuleDB` to find specifications with no more than a given number of
strategies of certain types
- log information when expanding a verified combinatorial class.
- added `is_equivalence` method to `Rule`
- sanity checking in multiple variables. In order to use this one must implement
the method `possible_parameters` on their `CombinatorialClass`. The sanity
checker only checks counts, not generation of objects.
- Added the `initial_conditions` method to `CombinatorialClass` and a
`get_initial_conditions` method to `CombinatorialSpecification`.

Fixed
- when subbing parameters use simultaneous flag
- Retrieving the rule in the forget db when the rules comes from applying a
strategy to a child.
- When a parameter does not map to equivalent child we don't look for it on the
child, preventing a `KeyError`.
- the extra parameters dictionary is flipped when creating the constructor in a
reverse rule.
- fixed the `EquivalencePathRule.constructor` method
- only save equivalence rules for rules a -> (b,) if a and b are equivalent.

Changed
- When the searcher finds a specification, it will now spends 1% of the time
spent searching trying to find a small specification instead of just returning
a random one.

Added
- When expanding a class with a strategy, you can now create rules where the
parent is not the class passed to the strategy.
- The 'get_equations' method now handles multiple parameters

Changed
- Removed some of the detailed timing in the queue to make status report
shorter.

Changed

This release is a major change of the `comb_spec_searcher` structure.
Instead of being centered around the `ProofTree` object, this version
introduces the `CombinatorialSpecification` object that offers many more
features. Most notably the support for arbitrary constructors, object building
and random sampling. The new version also widely improves support for multiple
variables.

Added
- `ProofTree.expand_tree()` is a method the can be used to expand a proof tree
using a strategy pack. This can be used to expand strategy verified
combinatorial classes, to give a single tree.

Changed
- The `CombinatorialSpecification.auto_seach()` only uses keyword arguments.

Fixed
- Fix the `is_expanded` function to check for inferral and initial expansion
- Fix the `__contain__` of ClassDB so that it actually works.

Removed
- Support for Python 3.5 and earlier