coefficient.[ch]

[Jason Stover]

I moved coefficient.[ch] to src/math since its routines
will be necessary in more than the regression procedure.

I have two questions:

- Should they be lib?
- Linking happens in this order on the command line:

  ./src/math/libpspp_math.a ./src/math/linreg/libpspp_linreg.a

  but I had to reverse that so libpspp_linreg.a knew about coefficient.[ch].
  I'm not sure how to change this in the automake.mk's, and I'm not
  sure it should be changed if coefficient.[ch] belong in lib.

Advice?

-Jason