pvlib.soiling.hsu takes tilt instead of surface_tilt

[kandersolar]

pvlib.soiling.hsu takes a tilt parameter representing the same thing we normally call surface_tilt:

pvlib-python/pvlib/soiling.py

Lines 13 to 14 in 7a2ec9b

	def hsu(rainfall, cleaning_threshold, tilt, pm2_5, pm10,
	depo_veloc=None, rain_accum_period=pd.Timedelta('1h')):

pvlib-python/pvlib/soiling.py

Lines 33 to 34 in 7a2ec9b

	tilt : float
	Tilt of the PV panels from horizontal. [degree]

I don't see any good reason for this naming inconsistency (I suspect tilt just got copied from the matlab implementation) and suggest we rename the parameter to surface_tilt with a deprecation.

Also, the docstring parameter type description says it must be float, but the model's reference explicitly says time series tilt is allowed:

The angle is variable for tracking systems and is taken as the average angle over the time step.

[kandersolar]

On second thought, I'm not seeing how this can be deprecated without reordering the parameters, which doesn't seem worth it to me. I'm inclined to rename it without deprecation in 0.10.0.