Keep references to input arrays in guru plan struct

[ludvigak]

This should hopefully avoid segfault due to users not knowing exactly how the memory management works under the hood.

Fixes #39

[codecov]

Codecov Report

Merging #40 (4568f59) into master (f935efa) will increase coverage by 0.35%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
+ Coverage   89.70%   90.05%   +0.35%     
==========================================
  Files           3        3              
  Lines         340      352      +12     
==========================================
+ Hits          305      317      +12     
  Misses         35       35              

Impacted Files	Coverage Δ
src/guru.jl	86.99% <100.00%> (+1.40%)	⬆️

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[tknopp]

Just tested this branch and can confirm, this fixes #39!

[ahbarnett]

looks good, haven't tested locally! Notice to fix "now" -> "know" in one of the comments. Thanks, Alex