Span (#24)

* Changing chebyshev implementation to remove numpy dependencies
* Update span.py

pina/chebyshev.py

@@ -1,7 +1,9 @@
-import numpy as np
+import torch
+
+
 def chebyshev_roots(n):
     """ Return the roots of *n* Chebyshev polynomials (between [-1, 1]) """
-    coefficents = np.zeros(n+1)
-    coefficents[-1] = 1
-    return np.polynomial.chebyshev.chebroots(coefficents)
-
+    pi = torch.acos(torch.zeros(1)).item() * 2
+    k = torch.arange(n)
+    nodes = torch.sort(torch.cos(pi * (k + 0.5) / n))[0]
+    return nodes