Check dimension of gram matrix

[till-m]

Resolves #85
Checks that the gram matrix supplied to svmpredict has dimensions (l, n) when predicting n items on l training vectors or dimensions (k, n) where k is the number of support vectors of the model. If neither is the case, it will throw DimensionMismatch.

I have marked this pull request as a draft since I haven't been using Julia for very long and would appreciate some oversight. Additionally, I think whether to allow (k, n) shaped matrices might warrant some additional discussion.

[codecov-commenter]

Codecov Report

Merging #87 (7e464f8) into master (b9f7a17) will increase coverage by 0.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage   86.56%   86.82%   +0.26%     
==========================================
  Files           6        6              
  Lines         253      258       +5     
==========================================
+ Hits          219      224       +5     
  Misses         34       34              

Impacted Files	Coverage Δ
src/LIBSVM.jl	93.42% <100.00%> (+0.22%)	⬆️

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[iblislin]

Thanks! 👍