RATE Function Permits Floating Point NPER

[oleibman]

Suggested by PR #848 from @markkimsal. The RATE calculation had already been corrected, so that part of the PR was unnecessary, however one of the tests included a floating point value for Number of Periods, which Excel permits. PhpSpreadsheet till now expected that parameter to be an integer. This is trivially changed with some tests added.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.