implement RATE algorithm from ODFF

[markkimsal]

This is:

- [*] a bugfix

Fixes #324

Implement the ODFF algorithm for RATE() from LibreOffice. Add test data that tests floating point $nper.

[markkimsal]

The scrutinizer checks is flagging too many things as unused code.

$found = false
while (!$found) {
  if ($check) {
      $found =true;
  }
}

It says that $found=true is unused. That and a few other variables that are definitely not unused.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
If this is still an issue for you, please try to help by debugging it further and sharing your results.
Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
If this is still an issue for you, please try to help by debugging it further and sharing your results.
Thank you for your contributions.

[oleibman]

Superseded by 4107.