inifcns_nstdsums.cpp: S_num takes cl_N as an argument instead of numeric.

inifcns_nstdsums.cpp: S_num takes cl_N as an argument instead of numeric.

Implicit conversion from cl_N to numeric considered harmful.

Using GiNaC::numeric for numerical computations incurs significant
overhead, so one might want to do these computations using proper CLN
types. Unfortunately, it's not easy due to automatic conversion from
cln::cl_N to GiNaC::numeric. Here is a simple example:

cl_N x, y;
// initialize them
return sin(x) +  y*exp(y);

The compiler complains about ambigously overloaded of functions, i.e.
cl_N cln::sin(const cl_N&) versus numeric GiNaC::sin(const numeric&).

Hence, I propose to disable *implicit* conversion from cl_N to numeric
(this can be done by marking the numeric ctor as `explicit').

However, this change happens to be a bit nontrivial, because that evil
conversion is used in GiNaC itself. So, I decided to rewrite those fragments
of code.