#6 Modify the default string used to signal help is required

Modify the default string used to signal help is required (and put one that should never be used).

Previously the default behaviour was to use '?' to that effect, but when cell ended by this symbol - typically in a comment - it was not correctly interpreted.

We tried first to overload metakernel get_kernel_help_on() to modify the behaviour when line was ending by a '?', but it was more complicated and I didn't manage to execute the code when the '?' was in fact in a comment.

A great thanks to @ldavid who found out the question mark was the cause of the weird behaviour and who found the final solution we devised!