Group knockoffs and efficiency improvements

[biona001]

• Supports single-chromosome group knockoffs for PLINK file (support for multiple chromosomes in a single plink file should not be far away)
• General efficiency improvements for group knockoffs
• Trying a O(n) formula for updating cholesky factorization in MVR, max_entroy, and sdp_fast knockoffs. With tol=0.001, MVR and max_entropy knockoffs should take <1h on 10k by 10k covariance matrix, and with tol=1e-6 (default), max_entropy should still be <1h while MVR should take slightly more than an hour.

[codecov-commenter]

Codecov Report

Merging #32 (f95f2c8) into master (b057a60) will decrease coverage by 0.43%. The diff coverage is 47.16%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
- Coverage   47.77%   47.34%   -0.44%     
==========================================
  Files          16       16              
  Lines         921     1016      +95     
==========================================
+ Hits          440      481      +41     
- Misses        481      535      +54     

Impacted Files	Coverage Δ
src/Knockoffs.jl	0.00% <ø> (ø)
src/ghost.jl	0.00% <0.00%> (ø)
src/group.jl	56.00% <0.00%> (-42.25%)	:arrow_down:
src/utilities.jl	60.06% <80.64%> (+3.53%)	:arrow_up:

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