Add a second check for checkpoint file format, in case it doesn't use default fields but variables instead

Provide ability to skip over solvers without variable specs, still an experimental feature, so may break in future

Add way to check checkpoint files. Checkpoint files from paramaeters have same format as fld file, those from filters don't, add experimental feature to support that

Add way to check whether checkpoint files are consistent with output files. Note, only for checkpoint files generated from parameters, not from filters

- Checkpoint behaviour if both specified in filters and parameters is to read both and ignore checkpoints as filter files

- Corrected APE Acoustic parameters

Filters now append appropriate extensions where needed

Fix bug so boundary condition reference file checks include ones for Dirichlet and Robin, previously only Neumann BC files were checked