Build 4.0.0.13 of xlsgen is a fix pack for the calculation engine.

A number of issues for fixing how to retrieve values in cells :

- better handling of "always calc" flag in cells
- fix for volatile functions
- fix for calculating external references when the corresponding file cannot be opened
- querying for a formula in a cell returns an empty string instead of NULL when there is no formula, avoiding needless exceptions
- better handling of shared formulas internally when a cell value is being updated
- ensure that DeleteFormulas() preserves hidden rows