added align_vector() and minor other changes

+ added an align_vector() function which is very similar to pad_vector
(and based thereupon) to make two vectors identical lengths, padding if
necessary
+ moved factorise_field() from datamat_tools (which is now empty and
deleted) to utils module
+ add_average_field() no longer requires span_start_idx or span_end_idx
to be present in the DataMat, but will use them if they are
+ added a get_short_function_name to pretty-up presentation of functions
esp. for storing as a variable