(34S) Integration (using the double-exponential method)

[emece67]

Thanks Pauli.

There's a little problem in the header you added. It says:

/* The exposed functions in this file don't use the normal prologue/epilogue
* and do not operate in double precision. They call back to user code and
* manage the stack, input, output and locals themselves.
*
* Changes to this will likely cause breakage.
*/

But it does work in double precision. In any case, it is not a big issue as this will not be read by common users.

Regards.