Make columns property optional on Worksheet

[MaxDesiatov]

As reported in #12, there are worksheets that apparently don't have cols node, which means that columns property on Worksheet should be optional.

[codecov]

Codecov Report

Merging #14 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #14   +/-   ##
=======================================
  Coverage   97.86%   97.86%           
=======================================
  Files           4        4           
  Lines         187      187           
=======================================
  Hits          183      183           
  Misses          4        4

Impacted Files	Coverage Δ
Sources/CoreXLSX/Worksheet.swift	100% <100%> (ø)	⬆️

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