Modularize constructors

[palday]

This splits our main (non default) constructor into several stages. The idea is that packages like unfold.jl that do a lot of the model matrix construction can still take advantage of the rest of our machinery and not duplicate effort in a brittle way.

@behinger

[palday]

I think this should be a patch release because these changes are officially marked as being "internal", even those some of our collaborators are going to use these internal changes as a versioning requirement.

[codecov]

Codecov Report

Merging #449 (cb755e3) into master (f0caa2b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #449   +/-   ##
=======================================
  Coverage   92.85%   92.86%           
=======================================
  Files          23       23           
  Lines        1736     1738    +2     
=======================================
+ Hits         1612     1614    +2     
  Misses        124      124           

Impacted Files	Coverage Δ
src/linearmixedmodel.jl	94.26% <100.00%> (+0.03%)	⬆️

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[dmbates]

Looks good. Thanks.