Filter for SnpData

[kose-y]

Added more functionalities for SnpData, as follows.

• Added the field src to SnpData to keep track of the source plink files for it.
• Added show(io::IO, x::SnpData), for simpler output for a SnpData.
• Added filter based on person_info and snp_info fields of SnpData. #24, #29.
• Reimplemented split_plink based on filter. The previous syntax is unaffected. It can now take a SnpData as an argument.

[coveralls]

Coverage decreased (-0.3%) to 95.067% when pulling 327bced5b9ac8492e6a1c05af945d614c0011a55 on kose-y:master into 67941f1dcf528707ae68560b4c2f61e8f269932d on OpenMendel:master.

[codecov-io]

Codecov Report

Merging #35 into master will decrease coverage by 0.32%. The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
- Coverage   95.39%   95.06%   -0.33%     
==========================================
  Files           6        6              
  Lines         434      446      +12     
==========================================
+ Hits          414      424      +10     
- Misses         20       22       +2

Impacted Files	Coverage Δ
src/SnpArrays.jl	100% <ø> (ø)	:arrow_up:
src/snpdata.jl	95.5% <91.66%> (-1.9%)	:arrow_down:

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