subsetting

[Konrad1991]

[x] define structs SubsetNumeric, SubsetInteger and SubsetLogical
[ ] create getter and setter for those structs
[ ] identify in R when variable is subsetted
[ ] first create symbol table
[ ] traverse again and find subsetted variables.
Store corresponding subsets for each expression
- the variable which is subsetted
- the expression defining the indices of the subset
* possible expressions to define indices:
* int, double, Numeric, Integer, Logical,
SubsetNumeric, SubsetInteger and SubsetLogical
but not a calculation
[ ] define SubsetManager
[ ] pass SubsetManager to EXPRXY
[ ] throw error in R if user tries to subset scalar