Checkpointing: Adding new tests to demonstrate segmented dimension sweeps
Checkpointing: Adding new tests to demonstrate segmented dimension sweeps

Adds three new tests that cover the three basic sweep types one might
segment, ie. execute in multiple operator invocations. Each compares
against a reference solution and against a full single-invocation sweep.

The three types are:
* One-sided, first order in time; stencil(t)=(1, 0)
* One-sided, second order in time; stencil(t)=(2, 0)
* Two-sided, second order in space; stencil(t)=(1, 1)

This largely demonstrates the current conventions used to prescribe the
segmented dimension limits and will inform discussions on #449.

opesci/devito,opesci/devito