Output as JSON doesn't handle feature score of Float.NaN

If any feature has a score value Float.NaN, then
- Output to Textbox as JSON, select Export Features
- the feature is included in the JSON array as a null value
- NullPointerException occurs when it is parsed (on New Window from output) (and this aborts all feature parsing, so none are created)

Currently it is not simple to create a feature with a NaN score, however it can be done by amending exampleFeatures.txt to add NaN as an extra column, then load this on to uniref50.fa.

Float.NaN scores would be more commonly used (for example when parsing Ensembl data) were it not for bug JAL-2060.