Constant check

[TaylorOshan]

Noticed that when I tried to manually add a constant I wasn't getting the appropriate exception. For instance, model1 below works (no manual constant) but model2 throws a singular matrix error (because of the manual constant and automatic constant) instead of the expected "x array cannot contain a constant vector; constant will be added automatically".

import numpy as np
from pysal.spreg.ols import OLS
import pysal

db = pysal.open(pysal.examples.get_path('columbus.dbf'),'r')
y = np.array(db.by_col("HOVAL"))
y = np.reshape(y, (49,1))
X = []
X.append(db.by_col("INC"))
X.append(db.by_col("CRIME"))
X1 = np.array(X).T
X2 = np.hstack((np.ones(y.shape),X1))

model1 = OLS(y, X1)
model2 = OLS(y, X2)

It looks like the constant_check function is missing its input of 'x' and to obtain the correct behavior the 'not' should be removed from the if statement as well.

It was suggested that I tag a spreg developer pertaining to this potential bug. @pedrovma @darribas