store list of non-NULL fields in dump/load

[stevengj]

As discussed for #1738, it would be improve robustness to also save a list of which fields are non-NULL.

That is, for each field that is dumped (e.g. f_u) it should save a NUM_FIELD_COMPONENTS * 2 array of boolean values (indicating whether e.g. f_u[c][cmp] != NULL).

Then, when loading the fields, you check this array against the fields that are allocated, and throw an error if they don't match. (Alternatively, if a field is not allocated but is present in the file, then we could allocate it and then load it.)