mdot: tmp test to make sure mdot is actually faster.

This is going to be replaced by an actual unittest that does not use timing.
mdot: better description in release notes.
mdot: release notes: two arguments are possible.
mdot: optimization is not optional anymore.
mdot: no tests for unoptimized `mdot` anymore.
mdot: be more explicit in the docs
mdot: follow Colmen more closely

- use the same function name.
- variables have the same name
- return the cost matrix (if requested) to make _multi_dot_matrix_chain_order
  testable
mdot: explicitly test the DP results.