Not all selectors give the correct number of samples

[PaulWAyers]

We need sensible default values for the grid/sphere size in some of the selection algorithms.

We should also allow for the case where the number of selected molecules is too small. We can (easily?) optimize sphere-size, for example, using the secant method (evaluating at the closest integer) so that the right number of molecules is selected.

(related): As suggested by @FarnazH , we should allow for sphere size to be dictated by the number of molecules, rather than just a size.