Revert "Revert "refs #7957. Add log scaling feature.""

This reverts commit a588b297a0d001ef12bcd74703003f7ec9c083ad.
refs #7957. Use older Qwt features for RHEL6.

Use range feature of Qwt to get the interval as the last implementation did not work cross-platform. Also fix the LineCurve type such that zeros can be ignored when in log scale.
Merge branch 'feature/7957_log_scale' into develop