Allow assignment into an xpose_data object while maintaining the class

[billdenney]

This PR allows assignment into an xpdb object like the following while maintaining the "xpose_data" class. It replaces #135 and #139 and is rebased to the current master.

xpdb$foo <- "bar"

or

xpdb[["foo"]] <- "bar"

[codecov]

Codecov Report

Merging #153 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #153      +/-   ##
=========================================
+ Coverage    94.4%   94.4%   +<.01%     
=========================================
  Files          40      40              
  Lines        3055    3058       +3     
=========================================
+ Hits         2884    2887       +3     
  Misses        171     171

Impacted Files	Coverage Δ
R/xpose_data.R	98.43% <ø> (ø)	⬆️
R/xpdb_edits.R	88% <100%> (+0.29%)	⬆️

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