Modify run_dft_opt function to return hessian and frequencies

[Leticia-maria]

TODO:

• Add a function that corrects imaginary (negative frequencies)
• Add tests

@bruno-ecl As of now, I have added the functionality to the run_dft_opt function, would we rather have a separate function instead or do you think the way it is right now works?

I added to the run_dft_opt because the good practice in computational chemistry is to proceed with frequency calculations after you optimize the geometry, so I thought it would be nice to have it already embedded with geometry optimization, which does mean that I could not have it as a separate function tho. 😄

[Leticia-maria]

This PR addresses #21 and #23

[Leticia-maria]

• As per @yiwenwang47 said, frequency calculations will be optional and requested by the user. (I will just add it into a separate function)