Almost a ternary conditional operator

This function has two purposes:
1. Loops of simple if-else statements can be avoided through vectorisation, providing a performance boost.

% Example:
% vif(eye(3), 4, 1)
% ans =
%
% 4.0000 1.0000 1.0000
% 1.0000 4.0000 1.0000
% 1.0000 1.0000 4.0000

Example 2:
% tic; for i=1:100000; if 1; 4; else 1+i; end; end; toc
% Elapsed time is 0.831261 seconds.
%
% truth = rand(1000000,1)< 0.5;
% tic; vif(truth , 4, 1+i); toc
% Elapsed time is 0.097463 seconds.

2. Syntax for individual if-else statements can be simplified, in a manner reminiscent of a C-style ternary conditional operator, at a slight performance loss.

Example 3:
% x = vif(condn, 3, 4);
% % is easier to type than
% if condn
% x = 3;
% else
% x = 4;
% end

Updates: char and infinite inputs now correctly handled.