Subsume shapley values into semi-value computation

[mdbenito]

#319 duplicates some of the methods in value.shapley. We will want to merge as much as possible of the latter into the semi-values, possibly just by calling semivalues() from within the methods in value.shapley.

As of 0.7.0, there are two pieces missing for this:

• TruncationPolicies for compute_semivalues. This is addressed in Interruptible samplers #321
• The fact that the general computation of marginal utilities which does not reuse the last computed value doubles the compute wrt. the original implementation. This should be solved by enabling caching, but this can be inconvenient since it relies on memcached. This would be little better if we had a default custom shared memory cache for local caching when using joblib, but ray users still would need to install memcached, unless we use a ray actor as cache, or any mechanism that ray makes available.