enable naming of simulation materials

[dkriegner]

partly addressing issue #140

naming of materials for simulations allows to give more meaning full names to repeated materials. this allows to more intuitively link the parameters of such layers in simulations.

Exemplary use:

import xrayutilities as xu
# load materials
GaAs_ = xu.materials.GaAs
AlAs_ = xu.materials.AlAs
AlGaAs80 = xu.materials.AlGaAs(0.20)

# define layers
sub = xu.simpack.Layer(GaAs_, float('inf'))
lay_AlGaAs80 = xu.simpack.Layer(AlGaAs80, 20000, relaxation=0., name="AlGaAs SL")
lay_GaAs = xu.simpack.Layer(GaAs_, 2000, relaxation=0., name="GaAs SL")
cap = xu.simpack.Layer(GaAs_, 3200, relaxation=0.0, name="GaAs cap")

# make superlattice structure
heterostructure = xu.simpack.PseudomorphicStack001(
    'Hetereostructure', sub + (lay_AlGaAs80 + lay_GaAs)*5 + cap)
print(heterostructure)

The corresponding output is then

Hetereostructure [
  Layer-GaAs (a: 5.6532, at0_Ga_4a_occupation: 1, at1_As_4c_occupation: 1, at0_Ga_4a_biso: 0, at1_As_4c_biso: 0, thickness: inf, ),
  Layer-AlGaAs_SL (a: 5.6532, at0_Al_4a_occupation: 0.8, at1_As_4c_occupation: 1, at2_Ga_4a_occupation: 0.2, at0_Al_4a_biso: 0, at1_As_4c_biso: 0, at2_Ga_4a_biso: 0, relaxation: 0, thickness: 20000, c: 5.6664, ),
  Layer-GaAs_SL (a: 5.6532, at0_Ga_4a_occupation: 1, at1_As_4c_occupation: 1, at0_Ga_4a_biso: 0, at1_As_4c_biso: 0, relaxation: 0, thickness: 2000, ),
  Layer-AlGaAs_SL_1 (a: 5.6532, at0_Al_4a_occupation: 0.8, at1_As_4c_occupation: 1, at2_Ga_4a_occupation: 0.2, at0_Al_4a_biso: 0, at1_As_4c_biso: 0, at2_Ga_4a_biso: 0, relaxation: 0, thickness: 20000, c: 5.6664, ),
  Layer-GaAs_SL_1 (a: 5.6532, at0_Ga_4a_occupation: 1, at1_As_4c_occupation: 1, at0_Ga_4a_biso: 0, at1_As_4c_biso: 0, relaxation: 0, thickness: 2000, ),
  Layer-AlGaAs_SL_2 (a: 5.6532, at0_Al_4a_occupation: 0.8, at1_As_4c_occupation: 1, at2_Ga_4a_occupation: 0.2, at0_Al_4a_biso: 0, at1_As_4c_biso: 0, at2_Ga_4a_biso: 0, relaxation: 0, thickness: 20000, c: 5.6664, ),
  Layer-GaAs_SL_2 (a: 5.6532, at0_Ga_4a_occupation: 1, at1_As_4c_occupation: 1, at0_Ga_4a_biso: 0, at1_As_4c_biso: 0, relaxation: 0, thickness: 2000, ),
  Layer-AlGaAs_SL_3 (a: 5.6532, at0_Al_4a_occupation: 0.8, at1_As_4c_occupation: 1, at2_Ga_4a_occupation: 0.2, at0_Al_4a_biso: 0, at1_As_4c_biso: 0, at2_Ga_4a_biso: 0, relaxation: 0, thickness: 20000, c: 5.6664, ),
  Layer-GaAs_SL_3 (a: 5.6532, at0_Ga_4a_occupation: 1, at1_As_4c_occupation: 1, at0_Ga_4a_biso: 0, at1_As_4c_biso: 0, relaxation: 0, thickness: 2000, ),
  Layer-AlGaAs_SL_4 (a: 5.6532, at0_Al_4a_occupation: 0.8, at1_As_4c_occupation: 1, at2_Ga_4a_occupation: 0.2, at0_Al_4a_biso: 0, at1_As_4c_biso: 0, at2_Ga_4a_biso: 0, relaxation: 0, thickness: 20000, c: 5.6664, ),
  Layer-GaAs_SL_4 (a: 5.6532, at0_Ga_4a_occupation: 1, at1_As_4c_occupation: 1, at0_Ga_4a_biso: 0, at1_As_4c_biso: 0, relaxation: 0, thickness: 2000, ),
  Layer-GaAs_cap (a: 5.6532, at0_Ga_4a_occupation: 1, at1_As_4c_occupation: 1, at0_Ga_4a_biso: 0, at1_As_4c_biso: 0, relaxation: 0, thickness: 3200, )
]