Simplify imports for most used classes

[aymgal]

Concrete example:

from herculens.MassModel.mass_model import MassModel
from herculens.MassModel.Profiles.epl import EPL
from herculens.MassModel.Profiles.shear import Shear
...
mass_model = MassModel([EPL(), Shear()])

would become

import herculens as hcl
...
mass_model = hcl.MassModel([hcl.EPL(), hcl.Shear()])