MAINT: Fix some cython variable declarations to avoid warnings with cython 0.20.

Cython 0.20 is picky about declarations that mix pointer and non-pointer variables.  For example, here are the warnings generated when cython 0.20 is run on `interpolate/_ppoly.pyx`:

```
$ cython _ppoly.pyx
warning: _ppoly.pyx:277:16: Non-trivial type declarators in shared declaration (e.g. mix of pointers and values). Each pointer declaration should be on its own line.
warning: _ppoly.pyx:277:21: Non-trivial type declarators in shared declaration (e.g. mix of pointers and values). Each pointer declaration should be on its own line.
warning: _ppoly.pyx:559:16: Non-trivial type declarators in shared declaration (e.g. mix of pointers and values). Each pointer declaration should be on its own line.
warning: _ppoly.pyx:559:20: Non-trivial type declarators in shared declaration (e.g. mix of pointers and values). Each pointer declaration should be on its own line.
warning: _ppoly.pyx:684:16: Non-trivial type declarators in shared declaration (e.g. mix of pointers and values). Each pointer declaration should be on its own line.
warning: _ppoly.pyx:684:21: Non-trivial type declarators in shared declaration (e.g. mix of pointers and values). Each pointer declaration should be on its own line.
```
Line 277 is:

```
    cdef double *wr, *wi, last_root, va, vb
```

This PR moves each pointer's `cdef` to its own line.