get_groups documentation

[jd-lara]

          The functionality in get_groups combined with groupby is very powerful and clever. However, the documentation is a bit lacking.

1. I understand how these capabilities are used in PowerAnalytics.jl. Let's take an example where you want to sum active_power for generators grouped by fuel type. We need this functionality merged ASAP. 100% agree and let's do it.
2. If this is to be a public interface that we expect users to use, the documentation needs improvement. At minimum, there should be a complete workflow example. Common questions will be, "what is the difference between groupby = :each and groupby = :all? What is an example of a function that should be passed? This leaves me with three conflicting opinions. (1) Leave it alone with the understanding that very few people will ever consider this. (2) Don't export this method. (3) Go all in and give complete documentation.
3. Once a power user understands what's going on here, they are likely to ask why we did all this. They might say, why not rely on existing packages, like this example

using DataFrames
julia> combine(groupby(DataFrame(get_components(ThermalStandard, sys)), :fuel), :active_power => sum)
5×2 DataFrame
 Row │ fuel                               active_power_sum 
     │ ThermalF…                          Float64          
─────┼─────────────────────────────────────────────────────
   1 │ ThermalFuels.NATURAL_GAS = 7              29.7472
   2 │ ThermalFuels.COAL = 1                     13.6381
   3 │ ThermalFuels.DISTILLATE_FUEL_OIL…          6.5
   4 │ ThermalFuels.OTHER = 14                    0.0
   5 │ ThermalFuels.NUCLEAR = 9                   0.849257

If we go all in on documentation, we should anticipate this question and address it.

Originally posted by @daniel-thom in #1197 (comment)