Assorted minor bugfixes

[davidorme]

This PR:

• Fixes #86 by handling empty cells and non-string cells in Summary metadata fields more cleanly.
• Fixes #87 by checking for actual data in field min and max values before attempting to update extents.
• Fixes #91 by wrapping output in report method in BaseField subclasses with FORMATTER push and pop calls.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (ffaf30b) 69.04% compared to head (41cc005) 69.20%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release/3.0.0 #92 +/- ## ================================================= + Coverage 69.04% 69.20% +0.16% ================================================= Files 12 12 Lines 3644 3663 +19 ================================================= + Hits 2516 2535 +19 Misses 1128 1128 ```

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