Add AtomCenteredBasisSet and AtomCenteredFunction to basis_set.py

[EBB2675]

The schema should cover the standard basis sets:

• main basis set
• AuxC type
• AuxJ type
• AuxJK type

In addition, user defined basis sets, such as:

%basis
   AddGTO 6
     D   1
      1         0.15100000000          1.0000000000
   end
   AddAuxCGTO 6
     D   1
      1         0.11021124893          1.0000000000
     F   1
      1         0.31714624829          1.0000000000
   end
end

[EBB2675]

This is what im currently working on:

• [x] Case 1: usage of standard basis sets for entire system
• [ ] Case 2: add different basis sets to different atoms
• [ ] Case 3: add basis functions to different atoms, while using a standard basis
• [ ] Case 4: an entirely user-defined basis set

@ndaelman-hu can you think of another scenario for atom centered basis sets?

[ndaelman-hu]

Case 2: add different basis sets to different atoms

My question here would be how you decide to tie basis sets to groups of atoms? We can either tie them 1-by-1, or provide a mapping between the system hierarchy to basis sets. Likely there are even more ways, but starting with 1 general purpose would already be great.