Add resampling arguments

[Luuk-Renden]

Problem

If MC resampling is desired, now a hardcoded conditional is needed to activate this every $N$'th optimization step:
resample=(i%N==0) (where $i$ is the optimization iteration)
The number of MC steps $n$ to be taken also requires addition of a line:
model.equilibrium_steps = n

Solution

This should ideally being done by passing arguments, something like:
resample_every = N
resample_steps = n

Defaulting to $N=iternum+1$ (no resampling) and $n=0$ (perform no extra MC steps).
---Note that setting $0 < N \leq iternum$ will have no effect if $n=0$. It is still preferable to have default $N$ as $iternum+1$ to avoid unnecessarily entering resampling loop.---

That way, first a calculation without optimization can be performed to obtain the autocorrelation time $\tau$ of the MC steps:
python src/Fermion_He_3D_variable_NN.py --iternum 0 --batch 5000
(fit exponential with decay rate $\tau$ to autocorrelation of data in output file energy_equilibration.txt)

This $\tau$ can then be used to determine $n$ for a calculation with optimization:
python src/Fermion_He_3D_variable_NN.py --iternum 1000 --batch 5000 --resample_steps n --resample_every N

[Luuk-Renden]

Has been added as described.