Use size_t for View indices to prevent potential integer overflow

[keichi]

Summary

• Change loop indices from int to size_t when accessing 2D Views in stats.cpp and io.cpp
• Prevents potential integer overflow when the total number of elements exceeds INT_MAX

Changes

• stats.cpp: Use size_t for indices i and j in corrcoef(CrossMap, Dataset, TimeSeries)
• io.cpp: Use size_t for n_rows, n_columns, and loop indices in load_csv()

Test plan

• C++ tests pass