ENH: add sigFpe::fillNan(char* buf, size_t count)

- simplifies use with other allocators (eg, memory pools).
  Can also be used with other containers.

  vectorField fld = ...;
  sigFpe::fillNan(fld.data_bytes(), fld.size_bytes());

COMP: inline sigFpe::ignore helper class

- now unused (may be removed in the future), but can avoid compiling
  code for it

COMP: missing sigStopAtWriteNow() definition for MSwindows