fpspreadsheet: Add interest rate calculation to complete financial functions supported by fpspreadsheet. Extract financial stuff into separate unit "financemath.pas" which will be proposed as a fpc feature request.

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3294 8e941d3f-bd1b-0410-a28a-d453659cc2b4