[New Class] create WieghtedFeaturesNeighborhood class
change the return type arg of find_stable_topk_neighborhood and find_stable_mins_neighborhood
[DEBUG] Handling non-active features situation
[New Feature] create tools to get topk words of weights
rename the functions
create notes for learning python's meta programming
update notes and codes
add notes
[New Features] sdf.find_col_ptrs / sdf.extend_zeros_cols / sdf.to_pamdas_df
[New Features] sdf.append_rows
[DEBUG] modify the typo of pandas
[DEBUG] solve the row index confliction problem
update notes
[New ipynb] add tutorial note for reading corpus
[New Features] sdf.find_row_ptrs /  sdf.append_rows
[New Featues] add new merge types in sdf.append_rows
update sdf.append_rows
[DEBUG] solving UnboundLocalError: local variable 'appended_smatrix' referenced before assignment
explore numpy objects / try to build the prototype of sparse matrix and its index
[New Feature] sdf.merge_sdf