Discover properly negative max values

[grzesiul]

In order to find maximal value in data series, 'max' should be initialized with guard value - the least possible value. Unfortunately, it's been initialized with Double.MIN_VALUE, which is positive number, so series with purely negative values are handled improperly.

Moreover, I've extracted 'effective[Min|Max]' methods to DRY code a bit and introduced similar changes - removing usages of Double.MIN_VALUE. Still, comparisons appear to be always true.

Works correctly for:

• min: -94; max: 10; series: -93.0, -95.0, Double.MAX_VALUE, -95.0, -95.0, 20.0, 8.0, -95.0, -91.0, -(Double.MAX_VALUE - 1), -91.0, -95.0, -115.0, -92.0, -93.0, -95.0, -95.0
• min: -94; max: -92; series: -93.0, -94.0, -95.0, -92.0