Update StandardNamesRules.rst: add _due_to_planetary_boundary_layer

[climbfuji]

Add process suffix _due_to_planetary_boundary_layer as proposed new suffix in StandardNamesRules.rst.

This isn't exactly describing a process (i.e. is different than due_to_convection), but people use this or the abbreviation due_to_PBL a lot. Should it be even longer, i.e. _due_to_planetary_boundary_layer_processes ?

[nusbaume]

I think my two cents here are that I would prefer the longer _due_to_planetary_boundary_layer_processes, or maybe something more specific like _due_to_planetary_boundary_layer_turbulent_mixing (although I certainly realize it might be hard to disentangle every process if it is coming from a PBL physics scheme).

The reason I don't think I would want just _due_to_planetary_boundary_layer is because the PBL is also a location, and I could easily see variables like X_in_planetary_boundary_layer, which I feel could cause some confusion even with the different prepositions.

Of course I am happy to hear other people's thoughts as well, especially if they disagree. Thanks!

[gold2718]

Is _due_to_PBL_processes a decent compromise between length and clarity? I don't see any guidance in the CF conventions.