Update fit.jl

[matthieugomez]

• Throw error in residuals only when residuals not saved
• Remove predict with fixedeffects since the method did not handle missing or interaction properly
• dof_tstat -> dof_residual
• Correct remaining StatsBase -> StatsAPI

[codecov-commenter]

Codecov Report

Merging #208 (23a1372) into master (2662649) will decrease coverage by 0.18%. The diff coverage is 76.66%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
- Coverage   96.08%   95.90%   -0.19%     
==========================================
  Files           8        8              
  Lines         588      586       -2     
==========================================
- Hits          565      562       -3     
- Misses         23       24       +1     

Impacted Files	Coverage Δ
src/FixedEffectModels.jl	100.00% <ø> (ø)
src/FixedEffectModel.jl	92.76% <72.00%> (-0.79%)	:arrow_down:
src/fit.jl	96.26% <100.00%> (+0.01%)	:arrow_up:

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