WIP: Resolve segfault when missing TimeIntegrationMethod for unsteady problem - Fixes #260

This MR adds an assertion to ensure that the code fails cleanly with a helpful error message when the TimeIntegrationMethod is not specified in the <SOLVERINFO> section of the input file for an unsteady problem.

Since the if statement that previously wrapped the block of code undertaking the time integration initialisation has been removed, additional comments have been added to mark the start and end of this code block. This is intended to aid understanding and future maintainability of this section of code.

This resolves issue #260 (closed)