Support parsing of big-endian & RGB files. Also refactoring.

[notZaki]

This PR started as an attempt to incorporate the additions from PR #18 but then it got out of hand.
Sorry about that.

With the PR, the package is now able to read big-endian data.
However, it still can not correctly write big-endian files.
A workaround is to change the transfer-syntax by

dcm_data[(0x0002, 0x0010)] = "1.2.840.10008.1.2.1"

and then dcm_write will save the data as little-endian.

This PR also changes the optional arguments to dcm_parse and dcm_write, so it can break older code.
The optional arguments are now keywords (described in the summary below).
Any suggestions to the keywords are welcome.

I want to manually test the PR on different machines before merging it and this will probably take me a week.
If there are no objections until then, then I will go ahead and merge.

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Summary of changes:

• Support parsing big-endian data (from Handle big-endian data #18)
• Refactor dcm_parse() into smaller functions
  • In particular: the meta info (which is always explicit VR and little endian) is parsed in a separate function while the body (which can be implicit VR or big endian) is parsed by a separate function
    • The explicitness/endianness is determined after reading the meta info. I think this makes the logic clearer than the previous version
• Shuffled the functions around in DICOM.jl
  • I prefer the main function to appear first, followed by the smaller/supporting functions. The old version had the smaller functions first, while the two main functions (dcm_parse and dcm_write) were at the end
• Fix some of my poor choices back in PR Get dcm_write() to work. Switch to using Dict for handling data. #22
  • Changed the variable names from camelCase to snake_case since that's the more julian style
  • Changed the optional arguments to dcm_parse. They are now all keywords and are as follows:
    • return_vr::Bool controls whether to return the VRs with the parsed data. Default: false
    • preamble::Bool controls whether the 128-bytes + "DICM" preamble is present. This was previously header. Default: true
    • aux_vr is a user-supplied dictionary which can supplement/override the dictionary in src/dcm_dict.jl. This was previously named dVR.
  • Changed the optional argument to dcm_write. It used to not be a keyword, but now it has the keyword aux_vr.
• Update readme so that it uses the newer/above syntax
• Fix a bug with parsing pixeldata
  • Previously, the pixel data had dimensions of xr=rows, yr=columns, where rows and columns are values from the dicom header. The updated version has the order swapped, i.e. xr=columns, yr=rows.
  • This was not caught in any of the previous tests because they all had the same number of rows and columns. However, on non-square images, the old version produces artifacts.
• Support RGB pixeldata
  • It currently assumes that the channels are seperated (i.e. Planar configuration = 1). If the channels are interlaced then the user will have to de-interlace them.
• Added a new test which is big-endian & RGB
• Added windows to the travis tests & replaced the code-coverage script with a simpler/default version

[codecov]

Codecov Report

Merging #38 into master will increase coverage by 1.66%.
The diff coverage is 90.8%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage    87.3%   88.97%   +1.66%     
==========================================
  Files           1        1              
  Lines         252      272      +20     
==========================================
+ Hits          220      242      +22     
+ Misses         32       30       -2

Impacted Files	Coverage Δ
src/DICOM.jl	88.97% <90.8%> (+1.66%)	⬆️

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