Merge advance and init benchmarks into single executable

Currently the timestep and initialization benchmarks use two separate executables. This is wasteful because the timestep benchmark must do a full initialization anyway; we currently don’t time this, and for bigger problems it can be expensive.

This replaces the two benchmarks with a single executable that times the initialization and then times the time advance, and puts the results in two separate files.

Requires utils PR 56. This is also built on top of the varying benchmark sizes work in PR #193