Testing new features in branch dppvfx-dev

It is important to test the core features of the vfx dpp sampler which is being integrated in the dppvfx-dev branch

Generally speaking, some efforts must be made regarding the description of the different functionalities through more detailed comments and more explicit variables

A "global" test is performed through the sampler itself (see here) but the internal features should be tested as well

In `finite_dpps.py`

To clean up the big FiniteDPP class I would put the following in exact_sampling.py

L329-345 of .sample_exact() in a dpp_vfx_sampler function
L473-505 of .sample_exact_k_dpp() in a k_dpp_vfx_sampler function

In `bless.py`

Write the mathematical meaning of "(eps, lambda)-accurate dictionary" somewhere and refer to specific Theorem/equation in the reference

estimate_rls_bless Test the accuracy of the RLS estimation L57 np.linalg.svd -> import scipy.linalg as la [...] la.svd ?
reduce_lambda X-> data ? D-> eps_lam_approx_of_L ?
[bless] Test the (epsilon, lamba) approx of the kernel in some way on some examples rls_oversample -> rls_oversampling_param ? tau -> rls_hat or rls_estimated ?

In `utils.py`

evaluate_L_diagonal Test that it actually computes the diagonal of the corresponding kernel, room was made here
stable_invert_root Need to write the documentation, see todo in docstring and test it, I can't really tell what it does Suggestions: L240-1
```
stable_eig = ~ np.isclose(..)
m = np.count_nonzero(stable_eig)
```
L246
```
if [eigenvec_thin.shape, eigenval_thin.shape] != [(n, m), (m,)]:
 raise ...
```
In vfx_sampling.py
compute_nystrom_dict compute_nystrom_dict -> compute_nystrom_dict_approx_of_L_kernel ? oversample->oversampling ? H_bless -> nb_iter_bless ?
estimate_rls_with_embedding the naming of the function and args is confusing estimate_rls_with_embedding -> estimate_rls_with/from_embedded_points ?
vfx_sampling_precompute_constants desired_s is not explicit enough -> ??? natural_s is not explicit enough -> mean_dpp_cardinality/size_estim ? f_opt where does this come from ?
vfx_sampling_do_sampling_loop Explain better the starting point of the loop

guilgautier / DPPy