Allow writing bad pixel json text directly to PV, add unit tests for NDPluginBadPixel

[jwlodek]

Similarly to what was done with the NDAttributes, this PR adds the option to set the bad pixel filename PV to either be the filepath to the json file as before, or to just set it with the json text directly. This allows for easier runtime manipulation of the behavior of the plugin.

The PR also adds unit tests for the bad pixel plugin.

• Update gitignore
• Update bad pixel filepath record to allow NELM to be passed in during loading.
• Update NDPluginBadPixel to allow for setting the filename PV to be either a path to a JSON file, or the json string itself
• Add unit tests for NDPluginBadPixel, for both previous (load from file) and new (load from string) functionality.
• Update example common plugins to reflect ability to update NELM of bad pixel filename record

Still needs to be tested with an actual live AD setup. For now only been tested in the unit test suite.