feature/empty_corner_tables

[Antonio-Velasco]

core:

• Added proposed na_depth to df_find_tables.
• Changed isna_above logic to be true if cells [ c to c + na_depth ] at r-1 are all empty.
• Changed isna_right logic to be true if cells [ c to c + na_depth ] at r are all empty.
• Changed isna_down logic to be true if cells [ r+1 to r + na_depth ] are all empty.
• Changed isna_corner logic tobe true if cells ([ c to c + na_depth ], [ r to r + na_depth]) are all empty.
• all these variables behaviour is the same as before for na_depth = 1
• Added a isghost_corner variable that checks if there is data on the same row but up to na_depth cells to the right
  -Updated result.append to include a cell when is considered a ghost cell even if empty.

test_core: Added unit test for feature