A small improvement to the help

Version 2.7.1 is a minor update with an improvement to the way the help works:

• When the help is used to provide a hint for a puzzle with pencil markings, the help is more likely to show how the pencilled candidates can be narrowed-down, instead of going straight to a follow-on step. This makes it easier to understand the logical reasoning for the follow-on step.
• This version also addresses a possible cause of some occasional crashes.