ISExcelCell; added Get and Set functions for Long values

The ISExcelCell class now includes functions to Get and Set Long values. Previously only Integer values were supported.

The following new functions are available:

• GetLong
• Will return 0 if the value is not numeric or outside the limits noted below.
• SetLong
• Will attempt to set the Cell to the Long value provided.

The following limits are applicable:

• Integer
• -2,147,483,648 through 2,147,483,647
• Long
• -9,223,372,036,854,775,808 through 9,223,372,036,854,775,807

Therefore, Long supports a greater range of numbers.

NOTE: If calling GetInteger and GetLong and the cell's value is outside the permitted range, 0 is returned. Trying to use SetInteger with a long value will fail with a compile error, so are easier to spot.