Numpy polynomial correction (#689)

* Changed the implementation of exercise 3 in python essentials to make it
faster, more pythonic and use techniques explained in the text

* Corrected a bug in Exercise 1 of the numpy chapter

The np.poly1d() method takes the coefficients in decreasing powers
so they need to be flipped (np.flip()) before making the call

Improved the test case as this error wasn't detected when using
identical coefficients.

Moved the tested value to a variable rather than hard coded

Made the code more pythonic using np.ones_like() rather than
np.empty(len(coef))

* typo coeffs -> coef

* Revert "Changed the implementation of exercise 3 in python essentials to make it"

This reverts commit a8c4795.

source/rst/numpy.rst

@@ -919,8 +919,7 @@ This code does the job
 .. code-block:: python3
 
     def p(x, coef):
-        X = np.empty(len(coef))
-        X[0] = 1
+        X = np.ones_like(coef)
         X[1:] = x
         y = np.cumprod(X)   # y = [1, x, x**2,...]
         return coef @ y
@@ -929,12 +928,13 @@ Let's test it
 
 .. code-block:: python3
 
-    coef = np.ones(3)
+    x = 2
+    coef = np.linspace(2, 4, 3)
     print(coef)
-    print(p(1, coef))
+    print(p(x, coef))
     # For comparison
-    q = np.poly1d(coef)
-    print(q(1))
+    q = np.poly1d(np.flip(coef))
+    print(q(x))
 
 
 Exercise 2