Add constrained synthetic test functions (pytorch#1832)

Summary:
Pull Request resolved: pytorch#1832

Adds the following engineering-problem test functions:
- PressureVesselDesign
- WeldedBeam
- SpeedReducer
- TensionCompressionString

Also cleans up / unifies some of the the test function code.

Reviewed By: SebastianAment

Differential Revision: D45821102

fbshipit-source-id: e256a08554f9f9891b65c7ddf22b2ee12810c5d4

botorch/test_functions/__init__.py

@@ -54,13 +54,17 @@
  Levy,
  Michalewicz,
  Powell,
+ PressureVessel,
  Rastrigin,
  Rosenbrock,
  Shekel,
  SixHumpCamel,
+ SpeedReducer,
  StyblinskiTang,
  SyntheticTestFunction,
+ TensionCompressionString,
  ThreeHumpCamel,
+ WeldedBeamSO,
 )
 
 
@@ -99,17 +103,21 @@
  "OSY",
  "Penicillin",
  "Powell",
+ "PressureVessel",
  "Rastrigin",
  "Rosenbrock",
  "Shekel",
  "SixHumpCamel",
+ "SpeedReducer",
  "SRN",
  "StyblinskiTang",
  "SyntheticTestFunction",
+ "TensionCompressionString",
  "ThreeHumpCamel",
  "ToyRobust",
  "VehicleSafety",
  "WeldedBeam",
+ "WeldedBeamSO",
  "ZDT1",
  "ZDT2",
  "ZDT3",