Updated XYZwriter to allow file mode

[jaclark5]

Fixes #3836

Changes made in this Pull Request:

• Add key word argument to XYZWriter to allow users to specify the mode with which to open an XYZ file (default is 'wt').

PR Checklist

• [X] Tests?
• [NA] Docs?
• [X] CHANGELOG updated?
• [X] Issue raised/referenced?

[codecov[bot]]

Codecov Report

Base: 94.33% // Head: 94.33% // No change to project coverage :thumbsup:

Coverage data is based on head (b037c19) compared to base (fa8b03b). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3840   +/-   ##
========================================
  Coverage    94.33%   94.33%           
========================================
  Files          193      193           
  Lines        24981    24981           
  Branches      3370     3370           
========================================
  Hits         23567    23567           
  Misses        1365     1365           
  Partials        49       49           

Impacted Files	Coverage Δ
package/MDAnalysis/coordinates/XYZ.py	92.81% <100.00%> (ø)

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