Add gram_sum

[blegat]

No description provided.

[coveralls]

Coverage increased (+0.9%) to 75.955% when pulling 8b2ad10 on bl/gram_sum into d28fbaf on master.

[coveralls]

Coverage increased (+0.9%) to 75.955% when pulling 8e78296 on bl/gram_sum into d28fbaf on master.

[codecov-io]

Codecov Report

Merging #100 into master will increase coverage by 0.85%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
+ Coverage   77.56%   78.42%   +0.85%     
==========================================
  Files          22       22              
  Lines         477      496      +19     
==========================================
+ Hits          370      389      +19     
  Misses        107      107

Impacted Files	Coverage Δ
src/gram_matrix.jl	84.9% <100%> (+8.43%)	⬆️

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