Add `mock` inputs and xmaps to `xmap`

[cynthiahqy]

• [x] mock named recode
• [x] mock collapse pairs
• [ ] specify number of columns and rows
• [ ] mock recode xmap
• [ ] mock collapse xmap
• [ ] mock split xmap
• [ ] mock mixed xmap
• [ ] mock links? -- pop weights
• [x] #103

References for documenting datasets:

• https://r-pkgs.org/data.html#sec-data-data
• https://github.com/tidyverse/dplyr/blob/main/R/data-bands.R
• https://github.com/tidyverse/ggplot2/blob/main/R/data.R
• https://stackoverflow.com/questions/2310409/how-can-i-document-data-sets-with-roxygen
• https://roxygen2.r-lib.org/articles/rd-other.html?q=keywords#datasets

[cynthiahqy]

From #104:

Two types of data transformations using xmaps:

• nomenclature transformation -- what you really need the tidy nomenclature format for!
• aggregation/disaggregation (a bit meh, until you resolve hierarchical discrepancies?)

Examples:

• Use state-country example for aggregation/disaggregation (i.e. hierarchical nomenclature)
• product groups? trade data? // cross-country ex-post harmonisation?

Candidate examples:

• https://jforbes14.github.io/eechidna/articles/imputing-census-data.html
• NAICS/trade data