BUG: interpolate: ensure fitpack work arrays are aligned to double boundaries

gfortran on some optimization levels (-O3) assumes that double-precision
arrays are aligned in memory on sizeof(double) boundaries.

The fitpack_surfit routine failed to properly ensure this, which results
to crashes in some cases.

Fixes gh-1310
TST: interpolate: add regression test for gh-1310
Merge pull request #2698 from pv/fix-1310

 BUG: interpolate: ensure fitpack work arrays are aligned to double boundaries