Added new routine.

Compare two one dimensional cells of strings and count the number of common strings. The
optional second argument returns a formated list of common variables.

*Example*

If we have:

A = {'A1'; 'A2'; 'A3'; 'A4'; 'A5'; 'A6'};
B = {'B1'; 'A2'; 'B3'; 'A4'; 'A1'};

then

[n,message] = common_strings_in_cell_arrays(A,B);

returns n=3 and message='A2, A4 and A1'.