When only using the nParticle cache format functionality in a scene, maya would not do that so when a saved file is opened, it will complained that the cache format is unknown.
To avoid the issue, a new dummy dependency node named "partioForceLoad" was added.
When present in the scene, maya will add the necessary plugin require directives in saved scene file.
In order to also work with scene saved using the "Export selection" method, this node is linked to the nParticle cache node when it is created
using the createPartioCache, attachPartioCache and attachPartioCacheEx MEL functions