fliphybrid root fix, nnidistance functions, PhyLiNC fixed network optimization

[coraallensavietta]

• Fixes root placement after flipping hybrid in cases when the original hybrid node cannot be the root
• new functions nnidistance and uniquennineighbors to explore the sNNI distance and sNNI neighborhood of a network.
• new function phyLiNC_fixednetwork to optimize the parameters of a network without changing its topology.
• small docstring updates

[codecov]

Codecov Report

Merging #148 (5b10965) into master (13aca07) will increase coverage by 0.09%.
The diff coverage is 98.46%.

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
+ Coverage   84.75%   84.85%   +0.09%     
==========================================
  Files          28       28              
  Lines       10925    10985      +60     
==========================================
+ Hits         9260     9321      +61     
+ Misses       1665     1664       -1     

Impacted Files	Coverage Δ
src/moves_semidirected.jl	98.67% <96.96%> (+0.39%)	⬆️
src/compareNetworks.jl	99.26% <100.00%> (+0.06%)	⬆️
src/phyLiNCoptimization.jl	93.75% <100.00%> (+0.07%)	⬆️

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[coveralls]

Coverage increased (+0.09%) to 84.852% when pulling 5b10965 on flipfix into 13aca07 on master.