Optimize spacia.R

[wooyongc]

Optimized spacia.R via data.table and Rcpp.

• Use of data.table instead of data.frame, and relevant changes to code to reflect data type change
• Changed script to avoid using transpose, which can be expensive
• Rcpp for bag construction instead of looping in R
• Reduced memory usage on exp_sender and pos_sender by removing names from the vector elements, and creating a separate variable, names_sender.
• Tested on CosMx Liver data, same mean beta produced for same set of inputs and random seed