Add the sum method for vclVector objects

[daydreamt]

Dear Mr Determan, thank you for your efforts with gpuR!

While using it I noticed there is no sum() method for vclVectors, so I went ahead and added one, based on the one for vclMatrix. Please let me know if I can do anything more, or differently, to get it merged.

[codecov-io]

Codecov Report

Merging #115 into develop will decrease coverage by 0.15%.
The diff coverage is 7.69%.

@@             Coverage Diff             @@
##           develop     #115      +/-   ##
===========================================
- Coverage    81.39%   81.23%   -0.16%     
===========================================
  Files           58       58              
  Lines        12399    12424      +25     
===========================================
+ Hits         10092    10093       +1     
- Misses        2307     2331      +24

Impacted Files	Coverage Δ
R/wrappers-vclVector.R	44.44% <0%> (-14.1%)	⬇️
R/methods-vclVector.R	71.34% <100%> (+0.08%)	⬆️
src/vienna_stats.cpp	72.96% <8.33%> (-1.77%)	⬇️

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[coveralls]

Coverage decreased (-0.2%) to 81.238% when pulling 7edb41c on daydreamt:develop into b6527e8 on cdeterman:develop.

[cdeterman]

Thanks for the contribution! If you could, I would appreciate you adding the unit tests as well for the new method. Given that sum is part of the Summary generic I would say add this to the tests/testthat/test_cpu_vclVector_math.R and tests/testthat/test_vclVector_math.R files.