Function for rescaling pixel data

[notZaki]

This PR adds a rescale!() function which will rescale the pixel values by taking into account the rescale intercept and slope.
This allows for negative pixel values even if they are stored as unsigned integers, i.e. Pixel Representation = 0.

Example usage of the function would be:

# Reading
dcm = dcm_parse(file)
rescale!(dcm)
# Writing
rescale!(dcm, :backward) # Undo's the previous rescale
dcm_write(newfile, dcm)

The new tests are in lines 152-157 of test/runtests.jl.
The rest of the changes in the tests are from applying JuliaFormatter and converting the older open(...); dcm_write(...); close() statements to the simpler dcm_write(...) format.

[codecov]

Codecov Report

Merging #41 into master will increase coverage by 0.09%.
The diff coverage is 88%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   88.46%   88.55%   +0.09%     
==========================================
  Files           1        1              
  Lines         286      297      +11     
==========================================
+ Hits          253      263      +10     
- Misses         33       34       +1

Impacted Files	Coverage Δ
src/DICOM.jl	88.55% <88%> (+0.09%)	⬆️

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