Improved tick location logic in ``LogLocator``

`~.LogLocator.tick_values` previously returned two ticks that were outside

the range ``{vmin, vmax}``. This has been updated to only return all ticks

for the decades containing ``vmin`` and ``vmax``. If ``vmin`` or ``vmax``

are exactly on a decade this means no ticks above/below are returned.

Otherwise a single major tick is returned above/below, and for minor ticks

all the ticks in the decade containing ``vmin`` or ``vmax`` are returned.