Enable complex mpc walk

[ShaneRozenLevy]

Ideally should be multiple PRs, but this PR:

• Fix MPC formulation to treat momentum as a decision variable
• Changed to trapazoidal collocation
• Provided interface for setting mode sequence
• Added constraint for initial state
• Added interface for setting initial guess for all decision variables
• Created gain struct and associated yaml
• Created reference generation class with creates a references for walking/running gait using gait specification, nominal stand, and com velocity keyframes.

Per file changes, broken into the 3 main goals for this PR: MPC Formulation systems/controllers/kinematic_centroidal_mpc/kinematic_centroidal_mpc.*: Fixed mpc formulation, uses mode sequence, and interface changes to use other changes multibody/kinematic/kinematic_constraints.cc: Added a negation to properly constraint world point evaluator to foot position decision variable systems/controllers/kinematic_centroidal_mpc/kinematic_centroidal_constraints.*: Reworked constraint for fixed mpc formulation examples/Cassie/kinematic_centroidal_mpc/cassie_kcmpc_trajopt.cc: heavy rework given new code

Quality of life examples/Cassie/cassie_utils.*: Add helper function for getting joint names examples/Cassie/kinematic_centroidal_mpc/cassie_kinematic_centroidal_mpc.*: Cassie specific child class of kinematic_centroidal_mpc for reduced copies code in for examples examples/Cassie/kinematic_centroidal_mpc/kinematic_centroidal_mpc_gains.yaml: Gains yaml systems/controllers/kinematic_centroidal_mpc/kinematic_centroidal_gains.h: Gains class

Reference generation systems/controllers/kinematic_centroidal_mpc/kcmpc_reference_generator.*: Class for generating simple reference trajectory systems/controllers/kinematic_centroidal_mpc/gait.*: Class used for reference generation gait specification examples/Cassie/kinematic_centroidal_mpc/cassie_reference_utils.*: Broke out IK code for nominal stand to its own file systems/controllers/kinematic_centroidal_mpc/reference_generation_utils.* Utility file for reference generation class

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This change is 

[ShaneRozenLevy]

examples/Cassie/kinematic_centroidal_mpc/cassie_kcmpc_trajopt.cc line 121 at r1 (raw file):

Previously, yangwill (William Yang) wrote…

although we try to keep our file structure consistent, it'd be nice to keep it all within the repo - which is best accomplished using paths relative to the repo. Generally, I've done this by always running binaries from the root of the repo - but open to other suggestions.

So I initially had a relative reference, but it didn't work with bazel run. At BDI we just had an environment variable for our workspace. I'll think a little more on this.