DataType is not set correctly when exported to xlsx

[bvenn]

Is your feature request related to a problem? Please describe.
When a cell contains "MAR3" with DataType = string, in Excel the cell data type is set to General instead of Text. Therefore it is converted to March-03.

Describe the solution you'd like
The datatype encoded within the model should be correctly converted to Excel types.

Describe alternatives you've considered
None

[omaus]

The problem is that, atm., we don't have Stylesheet.CellFormat incorporated in our model.
Excel's cell format (General, Text and so on) has nothing to do with our DataType but with those Stylesheet.CellFormats. Thus, we need to implement them to correctly set FsCells to Text (if we want them to be so), see #21.