#2010 - String issues in formula calculations

[swmal]

• During formula calculation, EPPlus does not handle double char Unicodes characters (starting with a high-surrogate char and ending with a low-surrogate char) correctly.
• The TEXT function should handle arrays as input and be able to return an array instead of a single value.
• The SORT function did not handle multiple indexes (second arg) correctly.
• The SWITCH function did not handle an array as the first argument.
• Fixed an issue that caused an Exception during calculation if an external workbook was not available.