Iterative search method for memory estimator

[mfep]

• Add support for iterative memory estimation
• Needed by Support for httomolibgpu's remove_stripe_fw httomo-backends#82

Checklist

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have made corresponding changes to the documentation

[dkazanc]

In HTTomo we also have another runner, a so-called sweep runner that performs a search/sweep over the set of given parameters. Because Paganin filter has a kernel of certain size we must take an area normally bigger or equal to the size of that kernel.
So in this function we try to estimate how large the vertical crop must be based on how many slices fit the GPU memory. This is still calling an older memory estimator and the associated test actually fails as well. So we need to change that bit accordingly. We just need to know that with given parameters how many vertical slices would fit, exactly what the iterative estimator can do. Can this be fixed please?