[Internals] Few simplifications

[lrnv]

Hey I tried to do a bit of simplifications of the fit() code, I hope its more readable like that.

[codecov]

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[lrnv]

@rimhajal This simplifies the code but also changes the structure of the output of the pohar perme fitting when there are covariates, in particular when there is several covariates.

Maybe you could tell me what you think of the output of :

f1 = fit(PoharPerme, @formula(Surv(time,status)~sex), colrec, frpop)
f2 = fit(PoharPerme, @formula(Surv(time,status)~(sex+stage)), colrec, frpop)

---

Edit: Wait a bit I'm trying other output formats, I will ping you when i am done

[rimhajal]

that way people can tell what dataframe refers to which group, this is good!

[rimhajal]

julia> x.age
0x0000000000000002

julia> x.keys
16-element Vector{@NamedTuple{sex::Symbol, stage::Int64}}:
#undef
#undef
(sex = :male, stage = 1)
(sex = :female, stage = 3)
(sex = :female, stage = 99)
(sex = :male, stage = 99)
#undef
#undef
#undef
(sex = :male, stage = 2)
(sex = :male, stage = 3)
(sex = :female, stage = 1)
#undef
#undef
#undef
(sex = :female, stage = 2)

there are some weird outputs though

[lrnv]

@rimhajal Now the docs are adapted so this should also be good to go.