Find free file-name

The function finds first unused name of a file in a required directory. The final name of the file consists of a name base appended by n digits (possibly with leading zeros) expressing an order of the new file possessing the same name base. The function is good for generating names of files containing results of an alternative processing of the same data.

Forms of calls:
~~~~~~~~~~~~~~
filename = freename(dirname,base);
% dirname = name of the directory, where to find a new name
% base = base of the new name (without extension)
filename = freename(dirname,base,n);
% n = number of figures to be appended (3 is default)
% filename = free name equals a base appended by 'n' figure characters

Examples:
~~~~~~~~
filename =[freename('Results','meas',2) '.txt'];
The name 'meas01.txt' will be sent to filename if there is no file
of the same base name 'meas' present in the subdirectory 'Results'.
The name 'meas04.txt' is returned provided 'meas03' exists in the
subdirectory 'Results'.
file = [freename('./', 'data') '.dat'];
returns 'data001.dat' if no file file with the base name 'data'
exists in the current directory.