Allow mutability of ExamplesTable

When using ExamplesTable to collect tabular parameter data, it may be useful to allow additions of columns to the table for a given row, e.g. to express a result value for given row. The non-affected rows would then be given default blank values for the new column.

Likewise, we could start from the content data (list of maps) and want to create a new table with modified content.

In other words, ExamplesTable could be used for both the string->content and the content->string transformations when implementing a step with tabular parameters.