added construct Prim from poscar function

[seshasaibehara]

• Added a static_method to Prim that constructs it from a given poscar_path
• Added tests for the same

[bpuchala]

Per our discussion, this is a useful method, but we want to make sure it is clear that the most direct represntation of a POSCAR is a Structure, while Prim includes specification of degrees of freedom. So the plan is to implement the following two methods:

1) make_prim_from_poscar: following the Prim constructor signature, but the occ_dof argument will default to just using the POSCAR atom type as the only occupant allowed on a site, or accept a map of POSCAR type to List of molecule types that will be allowed on sites occupied by that type.

2) make_structure_from_poscar: a method to construct a Structure from a POSCAR

[seshasaibehara]

• @bpuchala I added occ_dof argument when constructing prim_from_poscar as discussed
• I also added tests for the functions