[WIP] add function to create index column

[maxwell8888]

uses year_sector_data column
needs to be generalised to take argument to define which columns values
to use to index

[maxwell8888]

closes #99. I think Matt G is working on #47 so will wait for that to be finished until I strip the indexing code out of the figure functions - so that it is easy to check the functions can still produce the same output.

[codecov-io]

Codecov Report

Merging #111 into master will decrease coverage by 1.89%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master     #111     +/-   ##
=========================================
- Coverage   85.28%   83.39%   -1.9%     
=========================================
  Files          18       19      +1     
  Lines         571      584     +13     
=========================================
  Hits          487      487             
- Misses         84       97     +13

Impacted Files	Coverage Δ
R/add_index.R	0% <0%> (ø)

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[maxwell8888]

I am going to remove usemethod stuff from this because it will end up being more of a utility function so don't want it as a method