Make close actually save VTK files

[jipolanco]

One would expect close to actually write a VTK file to disk (especially since the documentation states so), but this is currently not the case.

This PR fixes this issue by making close equivalent to the old vtk_save function. Moreover, the close function is used instead of vtk_save throughout the code, tests and docs. The vtk_save function still works and is left for backwards compatibility.

Closes #148.

[codecov]

Codecov Report

Attention: Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.90%. Comparing base (dc4c34c) to head (d0c26b0).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/WriteVTK.jl	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
+ Coverage   96.87%   96.90%   +0.03%     
==========================================
  Files          15       15              
  Lines         863      873      +10     
==========================================
+ Hits          836      846      +10     
  Misses         27       27              

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[KnutAM]

Amazingly quick response and resolution. Thanks @jipolanco 🚀