Use readxl package for loading Excel files

[pwep]

This is the first step in addressing #54, to be merged into your developer branch.

readxl seems lightweight and quick to set up to replicate structures returned from openxlsx's read.xlsx()

• excel_sheets lists the worksheets
• modify handing of worksheet names
• wrap loaded data in data.frame to match return of openxlsx function read.xlsx()
• continue to ensure only specified columns are loaded
• devtools::tests() all pass

Writing Excel files is the next step, and will follow in a separate pull request tomorrow.

[codecov[bot]]

Codecov Report

Merging #55 into developer will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           developer      #55   +/-   ##
==========================================
  Coverage      69.86%   69.86%           
==========================================
  Files             79       79           
  Lines           9815     9815           
==========================================
  Hits            6857     6857           
  Misses          2958     2958           

Impacted Files	Coverage Δ
R/DfReadCrossedModalities.R	70.00% <100.00%> (ø)
R/ReadJAFROCNewFormat.R	76.92% <100.00%> (ø)
R/ReadJAFROCOldFormat.R	77.77% <100.00%> (ø)

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