'Initializes the class.
Parameters
length : Sympifyable
A Symbol or value representing the Beam\'s length.
elastic_modulus : Sympifyable
A SymPy expression representing the Beam\'s Modulus of Elasticity.
It is a measure of the stiffness of the Beam material.
second_moment : Sympifyable
A SymPy expression representing the Beam\'s Second moment of area.
It is a geometrical property of an area which reflects how its
points are distributed with respect to its neutral axis.
variable : Symbol, optional
A Symbol object that will be used as the variable along the beam
while representing the load, shear, moment, slope and deflection
curve. By default, it is set to ``Symbol(\'x\')``.'

def __init__(self, length, elastic_modulus, second_moment, variable=Symbol('x')):

    self.length = length
    self.elastic_modulus = elastic_modulus
    self.second_moment = second_moment
    self.variable = variable
    self._boundary_conditions = {'deflection': [], 'slope': []}
    self._load = 0
    self._reaction_loads = {}
