_impl, _e

[Brian Gough <bjg at network-theory dot co dot uk>]

I am going to make error handling uniform across the library so that
all error reporting will be through return codes (like the C-library).
This is to simplify the library.

It means that where there are _impl & _e versions of a function there
will only be a single function in future.  No other changes.

For safety the default behavior will still be to use GSL_ERROR to
abort() on errors but it will only supported for debugging purposes.
For production programs you'll turn off the abort() globally, rather
than using _impl & _e function to choose the error handling model on a
per-function basis.  

(Per-function error handling might come back in version 2.0 through a
GSLContext argument to all functions, which will be easier to
maintain.)

If this is a problem for anyone let me know.