Prepare for moving interpolation.{h,cc} to kernel/ + cleanup

NOTE: changed the interface to use STL Vector instead of lists
 set main return value type explicitly in configure.ac
Moved interpolation into kernel/
Merge branch 'fix.omalloc.configure.ac' of https://github.com/surface-smoothers/Sources into spielwiese

* 'fix.omalloc.configure.ac' of https://github.com/surface-smoothers/Sources:
  set main return value type explicitly in configure.ac