efficiency of multifit_funceval

[abuts]

the function multifit:@mf_class/private/multifit_funceval.m works with page_op through binary operations and funceval operations.
When these operations are applied to filebacked objects, each operation involves the scan of the whole backing-file. The scan and data IO is often the most expensive operation in evaluation.

It is relatively easy to write mutlifit_funceval-specific page_op, which would perform all filebacked operations in a single scan. This would substantially improve the speed of the operation. It is also possible to enable some specific optimization on pixel operations as e.g. background and foreground objects are often calculated on the same sqw object, so no internal pixel sorting is necessary.