More Precision for Float to String Casts

[oleibman]

Fix #3899. Supersedes PR #4476, which will be changed to draft status and closed if this PR is merged.

A standard cast from float to string in PHP can drop trailing decimal positions. This can lead to problems above and beyond the usual problems associated with floating point. See the superseded PR for a more complete explanation.

StringHelper::convertToString is changed for how it handles floats. It will now do separate casts for the whole and decimal parts, and then combine the results. This affects Cell::getValueString and Cell::getCalculatedValueString. Xlsx Writer will now invoke convertToString before writing a float to Xml. Ods Writer already uses getValueString, so no change is needed there. Xls Writer writes its float values in binary, so no change is needed there. Tests are added for all 3 writers.

Aside from fixing some problems, it might appear that this change introduces some new problems. For instance, setting a cell to 12345.6789 will now result in 12345.67890000000079 in the Xml. This difference is an illusion, merely a consequence of floating point rounding. If you run the following check under PhpUnit, it will pass:

self::assertSame(12345.6789, 12345.67890000000079);

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

[oleibman]

Scrutinizer "error" is, once again, a false positive caused by its inability to properly interpret @return string[]. It has been suppressed.