`MapQddotToAcceleration` and friends

[RussTedrake]

In kinematic trajectory optimization, we currently don't support qdot != v because we don't have MapQddotToAcceleration.

Request:

• MultibodyPlant::MapQddotToAcceleration()
• MultibodyPlant::MapAccelerationToQddot()
• bonus:
  • MultibodyPlant::MakeQddotToAccelerationMap()
  • MultibodyPlant::MakeAccelerationToQddotMap()

[sherm1]

The existing MapQdotToVelocity() and related functions are methods of System (and thus Diagram) so that they can be invoked on a Diagram without having to dig out a MultibodyPlant (MultibodyPlant just implements the underlying virtual).

It would be easiest just to add a local function to MultibodyPlant but nicer to add it at the System level with appropriate overrideable virtuals. WDYT?

[mitiguy]

Just to clarify: @sherm1 -- does it make sense to start by adding to MultibodyPlant?

[sherm1]

@mitiguy I think so, since MbP has to implement it anyway even if we add the System virtual. @RussTedrake would users likely have direct access to the MultibodyPlant?

[RussTedrake]

In my use case, I do have the MultibodyPlant. I think that's definitely sufficient for now.

[jwnimmer-tri]

@sherm1 is this is Paul's currently assigned task, please add it to the project board at a row consistent with its relative priority.