Specify VR and fix typo when parsing pixeldata

[notZaki]

These changes should fix the issues noted in #42 and #43, namely that:

• vr was not defined in determine_dtype()
• the function element() should've been replaced by read_element() in pixeldata_parse()

[codecov[bot]]

Codecov Report

Merging #44 into master will increase coverage by 0.03%. The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
+ Coverage   88.55%   88.59%   +0.03%     
==========================================
  Files           1        1              
  Lines         297      298       +1     
==========================================
+ Hits          263      264       +1     
  Misses         34       34

Impacted Files	Coverage Δ
src/DICOM.jl	88.59% <83.33%> (+0.03%)	:arrow_up:

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[notZaki]

Superseded by #45