AutoMinorLocator and and logarithmic axis

[cimarronm]

Adds logic to check the scale when generating the ticks for AutoMinorLocator rather than just assuming it is linear scale. Outputs a warning if scale is not linear with no minor ticks.

[mdboom]

Do we really want no ticks when on a non-linear scale, or just a warning.

Also, there may be other non-linear scales besides just log, some of which may be appropriate for this ticker. Maybe better to blacklist log than whitelist linear?

[cimarronm]

Makes sense, I'll change it to blacklist log rather than whitelist linear.