Set default values of input variables

A quick function for setting the default value of some variables

default is useful for setting the default value of an optional value.
For example,
a function
function c=add(a,b)
default('a',1);
default('b',2);
c=a+b;
gives the following results:
>> add()
ans =

3

>> add(2)

ans =

4

>> add(2,5)

ans =

7

default can assign ordinary variables, or it can assign fields of structs.

default counts empty variables as unset, and so will assign them a default value, unless the opposite behavior is specified with count_empty=0.

For example,
>> add([],4)

ans =

5

allows us to default the first input while specifying the second.

default works by executing 'isempty' and 'exist' in the caller workspace using the 'evalin' command. It uses 'assignin' to assign the value.