Workflow for mass addition feature

[justinngan92]

Problem: In bridges, often there are scenarios where loading (trucks etc) contributes mass to the overall system of the deck but not stiffness.

Solution: A function feature in ospgrillage that enable user to add mass, either directly (to specify elements) or indirectly (handled by ospgrillage to distribute mass to nodes for example).

TODO: A proposed function that follows the following procedure in loop can be ideal for when the model has been built (ran create_osp_model()) :

1. loop through elements to get each element mass.
2. For elements identified that is subjected to added mass, sum the mass of added mass with current element mass
3. Overwrite mass parameter of element() commands