Replace `pickle` with `dill` for serialization

[langestefan]

This replaces the python pickle module with the more robust dill (see dill), which is used by many projects that require serialization.

This will fix the serialization part of issue #72 and prevent more serialization issues in the future.

Models of this form will no longer fail to serialize:

import cvxpy as cp
import numpy as np
from cvxpygen import cpg
import dill as pickle

# setup model
m, n = 3, 2
x = cp.Variable(n, name="x")
A = cp.Parameter((m, n), name="A", sparsity=((0, 0, 1), (0, 1, 1)))
b = cp.Parameter(m, name="b")
d = cp.Parameter(m, name="d")
bd = cp.CallbackParam(
    shape=(m,), callback=lambda: d.value * b.value, name="bd"
)
problem = cp.Problem(cp.Minimize(cp.sum_squares(A @ x - bd)), [x >= 0])

# set values for parameters
np.random.seed(0)
A.value = np.zeros((m, n))
A.value[0, 0] = np.random.randn()
A.value[0, 1] = np.random.randn()
A.value[1, 1] = np.random.randn()
b.value = np.random.randn(m)
d.value = np.random.randn(m)

# solve
problem.solve()
obj = problem.value
print("Objective value:", obj)

# code gen
cpg.generate_code(problem, code_dir="nonneg_LS", solver="SCS")

from nonneg_LS.cpg_solver import cpg_solve

with open('nonneg_LS/problem.pickle', 'rb') as f:
    prob = pickle.load(f)

prob.register_solve('CPG', cpg_solve)

obj = prob.solve(method='CPG')
print("Objective value:", obj)