Reduce memory usage of fitPfsReferenceFlux

I want to use PCA to reduce memory usage of fitPfsReferenceFlux.

Though it takes several days with 500GB of memory
to perform PCA on 6040 of 1e5-dimensional vectors (spectra), the program does finish,
and it seems that the input 6040 vectors can be approximated by
linear combinations of 1024 basis vectors with RMS relative errors 1.6e-4.

The interpolator of flux models has been a function from the 4-dimensional parameter space
(Teff, log(g), metal, alpha) to 1e5-dimensional vector space (R^4 → R^1e5).
With PCA, I can replace it with a function R^4 → R^1024,
"R^1024" being the set of coefficients in linear combinations of the 1024 basis vectors.

I decide to use RBF to fit this function R^4 → R^1024.
(Another candidate is polynomials, but I am not sure I would be able do without overfitting
if I were to use polynomials.)

I use the following procedure to estimate interpolation errors:

For each x[i] of input ~6000 spectra:
    Make RBF from the input ~6000 spectra except x[i]
    Interpolate a spectrum y at the same parameter as x[i]
    rms[i] = sqrt(mean(square((y - x[i]) / x[i])))

error = sqrt(mean(square(rms)))

Below is the histogram of "rms" with the best hyperparameters. The RMS of "rms" is 2.6e-4

I can see that the increase of errors from PIPE2D-1060 is negligible.

I want this new PCA-based interpolation merged to the master branch.
This method will reduce memory usage to 1/6 of the current method.

The new code requires a new version of fluxmodeldata, which I have uploaded here:

[-https://hscdata.mtk.nao.ac.jp/hsc_bin_dist/pfs/fluxmodeldata-ambre-20230602.tar.gz-]
https://hscdata.mtk.nao.ac.jp/hsc_bin_dist/pfs/fluxmodeldata-ambre-20230608.tar.gz

To use the new fluxmodeldata, users have to run ./install.sh --prefix=/path/to/pfs-packages [--set=small] to
pre-compute ~6,000 or ~60,000 spectra from the PCA basis vectors included in the package.