Fix for Issue 1495

[oleibman]

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?

#1495 reports that ActiveSheet can change when calculation
involves jumping around between sheets.
Save index before calculation, restore after, add test.

[MarkBaker]

Thank you, especially as the changes I'm working on to improve named ranges and include named formulae could exaccerbate this problem