combining nchoosek with bincoeff

[Francesco Potortì]

The nchoosek source contains this comment:

## FIXME -- This function is identical to bincoeff for scalar
## values, and so should probably be combined with bincoeff.

However, the two functions behave differently even for scalar values:

- nchoosek only works for integer values, and is very fast
- bincoeff works for real values, and can handle big numbers without
  allocating much memory

So I see two changes to be done:

1) nchoosek should check for integer input scalar values, and either
   a) barf if not integer or
   b) call bincoeff to give a correct result
2) bincoeff could check for integer arguments and use the different,
   faster algorithm of nchoosek in this case

I can make all or part of these changes, if people think they are
allright.

At least 1a) should be implemented, in my opinion, and the FIXME comment
removed.

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