Add some support for formulas in templates

[meld-cp]

Linked to #176

This will convert a cell with value: '$=SUM(C{{$enumrowstart}}:C{{$enumrowend}})' to the formula cell: 'SUM(CX:CY)' where X is the starting row of the enumerable and Y is the ending row.

Please see the added test case and example in samples/xlsx/TestTemplateBasicIEnumerableFillWithFormulas.xlsx