Require Python 3.7 and new features

Require Python 3.7 and adds new features for active learning and multi-fidelity optimization, along with a number of bug fixes.

Compatibility

• Require PyTorch >=1.4 (#379).
• Require Python >=3.7 (#378).

New Features

• Add qNegIntegratedPosteriorVariance for Bayesian active learning (#377).
• Add FixedNoiseMultiFidelityGP, analogous to SingleTaskMultiFidelityGP (#386).
• Support scalarize_posterior for m>1 and q>1 posteriors (#374).
• Support subset_output method on multi-fidelity models (#372).
• Add utilities for sampling from simplex and hypersphere (#369).

Bug fixes

• Fix TestLoader local test discovery (#376).
• Fix batch-list conversion of SingleTaskMultiFidelityGP (#370).
• Validate tensor args before checking input scaling for more
  informative error messaages (#368).
• Fix flaky qNoisyExpectedImprovement test (#362).
• Fix test function in closed-loop tutorial (#360).
• Fix num_output attribute in BoTorch/Ax tutorial (#355).

Other changes

• Require output dimension in MultiTaskGP (#383).
• Update code of conduct (#380).
• Remove deprecated joint_optimize and sequential_optimize (#363).