
TODO for matter package
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Priority: High
- Change 'drop=NA' behavior to use 'drop=FALSE'
- Let 'dim<-' switch between 'matter_vec' and 'matter_arr'
- Add Logic operators for 'matter_vec' class

Priority: Medium
- Sparse vectors/matrices (both in-memory and on-disk)
- Add 'match' and '%in%' functions for 'matter_vec' class
- Add more data manipulation functions for 'matter_df'
- Add wrappers for reading csv, tsv, etc.

Priority: Low
- Add support for different data sources via @datasources slot
- Add 'readonly' parameter to constructors and as an accessor method
- Smart caching of frequently-accessed parts of data
