Change Private Constructors for ReplaceChannel, YCbCrReplaceChannel classes to Public

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In my application, for the below two filters, the filter definition comes first 
and later the channel type and image is provided.

AForge.Imaging.Filters\
1. ReplaceChannel()
2. YCbCrReplaceChannel()

It would help, if you simply change the current Private default constructor to 
Public, same as in HSLFiltering class etc.
(Right now, I am doing it myself but a recompile is required)

Also thanks a lot for providing the world a beautiful framework.

Regards,
--
Abhishek Johri
Tokyo

Original issue reported on code.google.com by absolute...@gmail.com on 21 Jan 2014 at 12:35