add_points() does not handle objective functions that output integers

[sjohnson-FLL]

In surrogate.py, in the add_points() method, an AttributeError exception is raised by calling fx.ndim (line 64) when fx is an integer.

Potential solutions:

1. On line 62, expand type-checking to include integers as well as floats and update comment on line 59 accordingly.
2. Update documentation here to specify that the eval method must return a float. The comments inside add_points() already reflect this, but the documentation on readthedocs.io does not.