Increase test coverage for some PivotGrid services

[ChronosSF]

The task includes the following files:

• IgxPivotAggregate - @MayaKirova - unit tests
• IgxPivotResizeHandleDirective - @mddragnev - onDoubleClick is not covered - probably a UI test
• pivot-grid-navigation.service.ts - @mddragnev - some navigation options are missing, could be unit tested
• pivot-resizing.service.ts - @mddragnev - max/min column widths are not covered
• pivot-header-row.component.ts - filterRemoved and other functions
• pivot-grid.component.ts - @mddragnev - generateFromData is completely missed, displayDensity too
• pivot-row-dimension-header-group.component.ts - @mddragnev - activeGroup getter is missed
• pivot-strategy - @mddragnev - try to pass getFieldValue through the formatAsDate flag
• pivot-row-dimension-content.component.ts - @mddragnev - toggleDimension is not hit

You can refer to the whole coverage result: https://coveralls.io/builds/46594872