Refactor skills - move logic out of inheritance hierarchy

[joseph-ireland]

• Depends on Change initial strain value in skills from 1 to 0 #15014

Split out the implementations of strain peaks, decaying values, and exponential weighted sums into their own functions apart from the Skill inheritance hierarchy.

In preparation for a PR to add a list of attributes for each hit object - see #14934

The simplest implementation of that would requre skills to have their own Process(DIfficultyHitObject) function in order to have access to current and cumulative strain. Leaving the inheritance hierarchy as is would result in a confusing ping pong of method calls up and down the inheritance hierarchy.

Note: This affects difficulty values. Some of the simplifications make it slightly awkward to keep the initial strain value as 1, so I change the starting value to zero - see the relevant commit below.

It doesn't make sense for strain to start at 1 anyway - it's basically an unconfigurable strain for the first hit object which isn't affected by SkillMultiplier

[bdach]

requesting review from committee wrt the initial strain change

[peppy]

Why is this adjusting values and also majorly changing structure? Why are those two things not done separately?

[bdach]

@peppy have already brought this up in #15010 (comment). i do agree it may be better to split off the initial strain change to a separate PR.

[bdach]

Several merge conflicts to be resolved here, unfortunately :(

[bdach]

The structure seems fine to me. A few nitpicks with the docs but I consider the blockers resolved.

[bdach]

Unsure if this is a better description than the previous one. They're both pretty barebones.

[joseph-ireland]

My thought was that the previous docs pretty much say public abstract class Skill in words. I can revert or even just delete the doc

blockers resolved

[GoldenMine0502]

I can't understand why straindecaybase and sectionlength should go into parameter.
why you did it? and what do we anticipate through it?

[smoogipoo]

Gonna close this for the time being since there's going to be some heavy merge conflicts. Feel free to rebase and open a new PR as I still think it's a good direction.