remove all_vars from argument of convert_all and decode_solution

[nariaki3551]

As per title, this change removed all_vars from the argument of convert_all and decode_solution functions.
I think that it is more useful for users not to have to provide all_vars, a list of dict variables, to these functions.

I have tested this pull request by pytest and the following code.

import pulp as pl
import numpy as np

# Variables * must be defined as dictionaries
x = {
    i: pl.LpVariable("x_{}".format(i), cat=pl.LpInteger)
    for i in range(2)
}

problem = pl.LpProblem()
problem += x[0] + x[1]
problem += x[0] + x[1] >= 2

import pulp2mat
from scipy.optimize import milp
c, integrality, constraints, bounds = pulp2mat.convert_all(problem)
result = milp(c, integrality=integrality, constraints=constraints, bounds=bounds)

pulp2mat.decode_solution(result, problem)
print("x[0] =", x[0].value())
print("x[1] =", x[1].value())

[rtonoue]

@nariaki3551,

Thank you! Getting variable properties from pulp.LpProblem seems to be more straightforward.
I will modify some comments and the readme accordingly to this change.

[nariaki3551]

Thanks for your review and merge!