Send correct XML for get_nvts

[wiegandm]

The get_nvts method previously sent get_notes instead of the correct GMP command.

This PR provides the unit test used for confirming this bug, the bug fix, a CHANGELOG entry and a unit test for the get_nvt (singular) method.

[codecov]

Codecov Report

Merging #84 into master will increase coverage by 1.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   58.11%   59.19%   +1.08%     
==========================================
  Files          10       10              
  Lines        2218     2218              
==========================================
+ Hits         1289     1313      +24     
+ Misses        929      905      -24

Impacted Files	Coverage Δ
gvm/protocols/gmpv7.py	54.92% <100%> (+1.39%)	⬆️

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