Restart from checkpoint with inconsistent box sizes

[KAClough]

Changing box sizes on a restart can lead to problems where finer levels get stuck and can't regrid. This happens for example if you started with a block factor of 16 and later increase it to 32. It is a particular issue for restarting from the IC solver, where the grid setup can be quite different.

This should be picked up when restarting and call a Mayday error.

Suggestion by @mirenradia:

In GRAMRLevel::readCheckpointLevel, we can iterate through the read Vector object (the one read in these lines: https://github.com/GRChombo/GRChombo/blob/4baece8164eef6fae5e1b29d929ef324d9e8081b/Source/GRChomboCore/GRAMRLevel.cpp#L660-L666) and check, for each box in the vector, that their longest side is <= max_box_size and block_factor divides the length of all sides.