Added data aggregation

[WhoCares4400]

Added data aggregation

Added data aggregation - values will be displayed on row group headers, aligned with columns.
User can specify the aggregation function for specific column from predefined functions or provide their own function.

Predefined aggregation methods that can be passed as string to "aggFunc" attribute:

• "avg" - average of all values in column
• "sum" - sum of values in column
• "count" - count of rows
• "max" - max value from all values in column
• "min" - min value from all values in column

Example:
https://codepen.io/Jakub-Turczyski/pen/MWdNxwY

New column attributes:

1. aggFunc - name of aggregation function or custom function provided by the user (REQUIRED)
2. aggDecimalPlaces - grouping aggregation default decimal places count (default 2)
3. aggShowTrailingZeros - defines whether the aggregated values should show trailing zeros or not (default false)

New "rowGroup" attributes:

1. aggPaddingLeft - grouping aggregation left padding value (default '10px')
2. aggPaddingRight - grouping aggregation right padding value (default '10px')
3. aggTextAlign - grouping aggregation default text alignment (default 'center')
4. aggDecimalPlaces - grouping aggregation default decimal places count - can be redefined for each column as well (default 2)
5. aggShowTrailingZeros - defines whether the aggregated values should show trailing zeros or not - can be redefined for each column as well (default false)

[AllanJard]

Very cool! Thank you for the PR. I'm travelling atm so I can't do a proper review, and I have some ideas that I'd like to use to align this with in DataTables core, so it might not make it in straight away, but I think this will be a great addition - thank you.