Add simpler percentile/RIC prelude example to Ch2

[Andrew Gene Brown]

Developing RIC for Loafercreek (2nd exercise in current chapter 2) is a bit involved, and may be too much to do before introducing EDA concepts, summary statistics, generalized horizons etc.

I am going to suggest adding in its place, or prior to it, some more discussion on basic profile-level summaries (and calculating them with user-defined functions/profileApply or existing aqp functions)

Also, to foreshadow RIC development from Ch 2, without going into the generalized horizon rabbit hole, perhaps using a simpler dataset, such as the example O horizon thickness data from Swanson (1993) in Soil Survey Horizons "Determining Ranges of Map Unit Characteristics: A Simple Method with a Statistical Basis" would be better. We could do an exercise using the Swanson data, or similar, then talk about how to calculate similar site-level properties by pedon/across horizons for use in these types of summaries, use in modeling, etc.

Developing the method in the way Swanson does in Ch 2 would allow the basic concept to be explored without getting into statistical method details--which is only briefly covered in the Ch 3 material at this time anyway.

It may be good to more formally build in the Generalized Horizon Label concept to Ch 2, in particular making the direct connection to the NASIS dspcomplayerid column (alias "genhz" in fetchNASIS()) and how it can be used for aggregation of pedon horizon data. That way it is introduced formally before doing an exercise, and before going into Ch 3 which pretty much presumes the student understands what they are and why they are using them for the various EDA exercises.