Fix opendap extension module.

- Revert back to unsigned chars for string arguments of str2<type> methods.
  Having a sign screws up the decoding of the values.

- Create new str2float32, str2float64, float32toStr, float64toStr methods to
  handle the floating-point arrays, instead of shoehorning floats through the
  integer methods.

- Use "s#" for PyArg_ParseTuple and Py_BuildValue on strings, to get/set the
  proper length of strings that may contain null characters.

- Remove limits on dimensions for PyArray_ContiguousFromObject, since we could
  be converting a multidimensional array.