Snp lin alg matrix mul

[biona001]

This PR

• Implements AX and A'X multiplications for A a SnpLinAlg and X a numeric matrix
  • Both tiling operations and unrolling inner loops are supported
  • Support impute/center/scale keywords and additive/dominant/recessive models
• Fixes #90 and #91
• Updates CI badge, remove travis, adds CITATION.bib
• Some testing and updated documentation (deleted a lot of mentioning of SnpBitMatrix)

If @kose-y thinks it's okay, and version 0.20.19 of VectorizationBase.jl is released, I think it's ready to merge

[biona001]

I also just realized the latest VectorizationBase.jl release is still at v0.20.18 (although for some reason Chris Elrod released v0.18.15 a few hours ago.... not sure why).

Need to wait until v0.20.19 is released before merging this PR. To test the code, one need to use master branch VectorizationBase.jl

[biona001]

VectoriztionBase.jl v0.20.19 has been released, so this pr is ready for review now.

[kose-y]

Thank you. It will be merged after re-running the CI tests.

[codecov-commenter]

Codecov Report

Merging #94 (1e54b67) into master (07b8d67) will decrease coverage by 1.36%. The diff coverage is 80.82%.

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
- Coverage   87.54%   86.18%   -1.37%     
==========================================
  Files          14       14              
  Lines        1285     1433     +148     
==========================================
+ Hits         1125     1235     +110     
- Misses        160      198      +38     

Impacted Files	Coverage Δ
src/linalg_direct.jl	86.31% <80.82%> (-8.35%)	:arrow_down:
src/vcf2plink.jl	95.16% <0.00%> (-0.23%)	:arrow_down:
src/SnpArrays.jl	100.00% <0.00%> (ø)

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