Avoid useless imports

[yguclu]

Problem Description

Importing modules from SymPDE is slow and should be speeded up.

One of the reasons is that some of our modules import lots of code from third-party libraries, which is not needed in most cases. For example, file sympde/utilities/utils.py starts with

import numpy as np
import itertools as it
from sympy import lambdify


from mpl_toolkits.mplot3d import *
import matplotlib.pyplot as plt

where the 3D matplotlib imports are only needed by the plot function plot_3d in the module, and not by lambdify_sympde. Moreover, there is no need to import everything with import *.

Suggested Solution

• Do not use: from ... import *
• Move slow imports into function definition