ENH: Add binary to compute energy contributions

[flokno]

The logic to evaluate energy contributions from force constants is there in 2 place right now:

• Right in main.f90 in https://github.com/tdep-developers/tdep/blob/main/src/extract_forceconstants/main.f90#L371
• As a module in anharmonic_free_energy: https://github.com/tdep-developers/tdep/blob/main/src/anharmonic_free_energy/epot.f90

To calculate the energy contributions per time step, currently the only way is to call extract_forceconstants --verbose, which will fit the force constants, evaluate the contributions, and write them to outfile.energies.

Suggestions:

• move the epot.f90 module to libolle
• import it in extract_forceconstants and anharmonic_free_energy
• potentially add a new binary just for evaluating the energies so that given force constants can be evaluated for separate samples/trajectories
• or alternatively make this available in anharmonic_free_energy