Fix some time rounding problems

[etimberg]

Fixes the issue seen in #2034

New Time Step Option

Adds a unitStepSize option to allow configurable increments of the base units. Setting this will override the automatic system. For example, to have tick marks every 3 seconds, the config would look like

scaleConfig = {
    time: {
        unit: 'second',
        unitStepSize: 3
    }
}