Improve spline efficiency and correct unexpected behavior

[ajacoby9]

Splines are very slow and occasionally have unexpected behavior. To reproduce the error:

import torch
from pina.model import Spline
from scipy.interpolate._bsplines import BSpline
from matplotlib import pyplot as plt

seed = 1999
torch.manual_seed(seed)

x_min, x_max = 0,1
x = torch.linspace(x_min, x_max, 50) 
order = 3
knots = torch.linspace(x_min, x_max, 10)
n_coeffs = len(knots) - order 
control_points = torch.randn(n_coeffs)
print(control_points)
model = Spline(knots=knots, control_points=control_points, order=order)
y = model(x)

spline = BSpline(
    t=knots.detach().numpy(),
    c=control_points.detach().numpy(),
    k=model.order - 1,
)
y_scipy = spline(x).flatten()
y = y.detach().numpy()
plt.plot(x,y, label="PINA", marker="o" )
plt.plot(x,y_scipy, label="SciPy", marker="x")
plt.legend()
plt.show()