Wrap a long syms equations

wraps a long syms equation so that it becomes more readable.

WRAP_string(string1) rewrites a long syms equation and wraps
it so that it becomes more readable. It automatically divides the expression into individual lines with three dots "..." at the end of each line. Moreover, it detects the existence of operators +,-,*,/ so that it doesn't cut a long name of a variable (such as Lamda_w).

Inputs:
string1: An expression written in syms
n_columns: users can change the maximum width of each line