fpspreadsheet: Allow to register user-provided functions for calculation of rpn formulas for which fpspreadsheet does not have a built-in calculation method (example: financial functions). Add example "test_formula_func" in folder "other". 

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3270 8e941d3f-bd1b-0410-a28a-d453659cc2b4