Fix warning about BOZ constant outside of DATA.

It's a bit weird that the standard doesn't even allow BOZ constants in
parameter initializers either.
added option EXACT_UNDOING_TO_DISK, which saves the forward run to disk in order to compute exact sensitivity kernels without having to undo the forward run numerically; it will read it back from disk instead. This is for validation purposes only, since the disk files will typically require tens of terabytes (yes, tera). However this will become affordable uin a few years.