CALCULATION_REGEXP_CELLREF is not sufficiently robust.

[oleibman]

It treats some perfectly legal defined names, e.g. A1A, as cell refs.
When the Xlsx Writer tries to save a worksheet which uses such a name
in a formula in a cell, it throws an exception.
The new DefinedNameConfusedForCellTest is a simple demonstration.
The Regexp has been changed to ensure the name starts on a Word boundary,
and to make sure it is not followed by a word character or period.
This fixes the problem, and does not appear to cause any regression
problems in the test suite.

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?

Bug fix, as described above. I am not sure how to go about adding a summary of the change to
CHANGELOG.md - that seems to be something that the project maintainer would do.
No documentation change is required.