Bug 410: Implement worksheet and workbook protection

[kainhofer]

-) Worksheet and workbook protection settings are properly read from .xlsx files with loadWorkbook and retained when saving with saveWorkbook -) Added functions protectWorkbook and protectWorksheet to implement workbook and worksheet protection with an optional password. -) Unit tests for testthat are included

[codecov[bot]]

Codecov Report

Merging #411 into master will increase coverage by 0.15%. The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #411      +/-   ##
==========================================
+ Coverage   60.23%   60.38%   +0.15%     
==========================================
  Files          30       30              
  Lines        7031     7106      +75     
==========================================
+ Hits         4235     4291      +56     
- Misses       2796     2815      +19

Impacted Files	Coverage Δ
R/helperFunctions.R	75.56% <100%> (+0.62%)	:arrow_up:
R/worksheet_class.R	73.28% <50%> (-0.37%)	:arrow_down:
src/load_workbook.cpp	90% <66.66%> (-0.15%)	:arrow_down:
R/wrappers.R	42.57% <68.18%> (+0.93%)	:arrow_up:
R/loadWorkbook.R	69.56% <75%> (-0.02%)	:arrow_down:
R/WorkbookClass.R	55.56% <84.61%> (+0.3%)	:arrow_up:

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[kainhofer]

This PR is superseeded by #413, which includes the implementation for issues #409 and #410 as well as workbook-level protection (with or without password).