New class: diffusionModelTxt

Create a new class "diffusionModelTxt", which will read a file to create the second order term.

assigned to @harry.cousin

created branch 2-new-class-diffusionmodeltxt to address this issue

created branch new-classdiffusion-modeltxt to address this issue

created branch 2-new-class-diffusionmodeltxt-2 to address this issue

created branch classdiffusion-modeltxt to address this issue

mentioned in merge request !3 (merged)

mentioned in commit edb18e31

closed with merge request !3 (merged)