Using dof approach to proper allowrankdeficient values

[Nosferican]

This PR fixes the dof for allowrankdeficient models and also dof_residual through the use of dof.

[codecov-io]

Codecov Report

Merging #265 into master will decrease coverage by 0.17%.
The diff coverage is 50%.

@@            Coverage Diff            @@
##           master    #265      +/-   ##
=========================================
- Coverage   51.97%   51.8%   -0.18%     
=========================================
  Files           6       6              
  Lines         583     583              
=========================================
- Hits          303     302       -1     
- Misses        280     281       +1

Impacted Files	Coverage Δ
src/lm.jl	50% <0%> (-1.14%)	⬇️
src/linpred.jl	48.31% <100%> (ø)	⬆️

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[nalimilan]

Thanks!

[Nosferican]

This is ready to be merged.