func arg for has_expr for custom equality check

[machow]

addresses #218 , by adding a func argument to SCTs like has_equal_value.

E.g. Ex().check_object('x').has_equal_value(func = lambda x, y: x == y)

Or with numpy

from numpy import allclose
Ex().check_object('x').has_equal_value(func = lambda x, y: allclose(x, y, atol = .01))

# or without atol option
Ex().check_object('x').has_equal_value(func = allclose)

[filipsch]

LGTM; clean!