Validate XIRR inputs and return correct error values

[blacknell]

Fix: Return #NUM! if values and dates contain a different number of values
Fix: Return #NUM! if there is not at least one positive cash flow and one negative cash flow
Fix: Return #NUM! if any number in dates precedes the starting date fixes #1120
Fix: Return #NUM! if a result that works cannot be found after max iteration tries
Fix: Correct DocBlocks for XIRR & XNPV
Add: Validate XIRR with unit tests

This is:

- [X] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• 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
• Documentation is updated as necessary

Why this change is needed?

If the date order is non ascending then numerous warnings are generated and the wrong result returned. In addition, a number of input cases result in the wrong error value being returned.

[PowerKiKi]

Thanks !