fpspreadsheet: Complete rpn calculations (COUNTA). Remaining missing function probably will not be implemented because calculation is beyond units (math, strutils, dateutils; if needed they can still be implemented at application level by registering the corresponding function).

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3296 8e941d3f-bd1b-0410-a28a-d453659cc2b4