Allow to select the output directory of executeOxcalScript() (fixes #42)

[nfrerebeau]

This should allow to choose where to write the output files (fixes #42).

my_oxcal_code <- ' Plot()
 {
  Sequence("Sequence1")
  {
   Boundary("Beginn");
   Phase("Phase1")
   {
    R_Date("Lab-1",5000,25);
    R_Date("Lab-2",4900,37);
   };
   Boundary("Between");
   Phase("Phase2")
   {
    R_Date("Lab-3",4800,43);
   };
   Boundary("End");
  };
 };'

# Write files named "results" in the working directory
my_result_file <- executeOxcalScript(my_oxcal_code, file = "results")

# Write files in the "here" directory (will be created if it does not exist) in the user home
my_result_file <- executeOxcalScript(my_oxcal_code, file = "~/here/results")

[codecov-commenter]

Codecov Report

Merging #47 (f16259a) into master (109a20d) will decrease coverage by 0.18%. The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
- Coverage   66.22%   66.04%   -0.19%     
==========================================
  Files          13       13              
  Lines         909      913       +4     
==========================================
+ Hits          602      603       +1     
- Misses        307      310       +3     

Impacted Files	Coverage Δ
R/executeOxcalScript.R	72.72% <40.00%> (-27.28%)	:arrow_down:

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[MartinHinz]

Many thanks for this helpful enhancement! Some tests fail, but that's not because of the new code, so I'll gladly include the pull request in the current development version.