Test and document interactions with decimal module context

[jagerber48]

The documentation indicates that sciform rounds using round-half-to-even rounding. This is true by default, but will be changed if the user modifies the decimal module context rounding option. This behavior should be documented and tested.

It is worth considering the following options:

• Explicitly configure sciform to always use decimal.ROUND_HALF_EVEN to avoid implicit coupling with users configuring the decimal context
• Extend the above proposal to include a user-configurable rounding strategy. This would provide more features, but it would also add even more configuration.

Likewise, the configurable decimal module precision can affect sciform formatting results.