ENH: Add vtkWordCloud to Infovis/Core

vtkWordCloud is an Image Source that creates a word cloud.

Word Clouds, AKA Tag Clouds, are a text visualization technique that displays individual works with properties that depend on the frequency of a word in a document. Numerous options are available, including the color of the words, size of the words, the orientation of the words, font files, and mask files. Word Clouds, AKA Tag Clouds, are a text visualization technique that displays individual works with properties that depend on the frequency of a word in a document. vtkWordCloud varies the font size base on word frequency.

Word Clouds are useful for quickly perceiving the most prominent terms in a document. Also, Word Clouds can identify trends and patterns that would otherwise be unclear or difficult to see in a tabular format. Frequently used keywords stand out better in a Word Cloud. Common words that might be overlooked in tabular form are highlighted in the larger text, making them pop out when displayed in a word cloud.

There is some controversy about the usefulness of word clouds. Their best use may be for presentations. Word clouds can be used to "compare" texts from similar subjects, e.g., Presidential Inaugural Addresses, job candidate comparisons, etc.

Several methods are available to customize the resulting visualization. The class provides defaults that provide a reasonable result.

Here are a few examples.