Add 'close' function to reader class

[RJMW]

This makes it easier to close the file_object handle https://github.com/pymzml/pymzML/blob/7f01356d409b77c809eb751431064ff5c5ba64bc/pymzml/run.py#L114 and avoids ResourceWarning:unclosed file errors when you loop through a list of mzml files or when you 'reload' the reader object.

[codecov[bot]]

Codecov Report

Merging #170 into dev will increase coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff            @@
##              dev    #170      +/-   ##
=========================================
+ Coverage   83.19%   83.2%   +0.01%     
=========================================
  Files          32      32              
  Lines        3505    3543      +38     
=========================================
+ Hits         2916    2948      +32     
- Misses        589     595       +6

Impacted Files	Coverage Δ
pymzml/run.py	86.06% <100%> (-0.44%)	:arrow_down:

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[MKoesters]

Perfect, thank you :)