Support Numo Gem for performing SVD

[mkasberg]

Background:
The slow step of LSI is computing the SVD (singular value decomposition)
of a matrix. Even with a relatively small collection of documents (say,
about 20 blog posts), the native ruby implementation is too slow to be
usable (taking hours to complete).

To work around this problem, classifier-reborn allows you to optionally
use the gsl gem to make use of the Gnu Scientific
Library when performing matrix
calculations. Computations with this gem perform orders of magnitude
faster than the ruby-only matrix implementation, and they're fast enough
that using LSI with Jekyll finishes in a reasonable amount of time
(seconds).

Unfortunately, rb-gsl is
unmaintained -- there's a commit on main that makes it compatible with
Ruby 3, but nobody has released the gem so the only way to use rb-gsl
with Ruby 3 right now is to specify the git hash in your Gemfile. See
SciRuby/rb-gsl#67. This will be increasingly
problematic because Ruby 2.7 is now in security
maintenance
and will become end of life in less than a year.

Notably, rb-gsl depends on the
narray
gem. narray is deprecated, and the readme suggests using
Numo::NArray instead.

Changes:
In this PR, my goal is to provide an alternative matrix implementation
that can perform singular value decomposition quickly and works with
Ruby 3. Doing so will make classifier-reborn compatible with Ruby 3
without depending on the unmaintained/unreleased gsl gem. There aren't
many gems that provide fast matrix support for ruby, but
Numo seems to be more actively
maintained than rb-gsl, and Numo has a working Ruby 3 implementation
that can perform a singular value decomposition, which is exactly what
we need. This requires
numo-narray and
numo-linalg.

My goal is to allow users to (optionally) use classifier-reborn with
Numo/Lapack the same way they'd use it with GSL. That is, the user
should install the numo-narray and numo-linalg gems (with their
required C libraries), and classifier-reborn will detect and use these
if they are found.

[mkasberg]

@mattr- appreciate all the PR reviews you've already done for me! I still have this PR marked as a draft, but I think it's ready for an initial round of feedback if you have time to do a review. Here are a couple things to focus on:

• We were previously using a global $GSL to indicate whether to use the gsl gem. I'm currently taking a similar approach, replacing that $GSL boolean with $SVD, which can be one of :ruby, :gsl, or :numo. I'm open to suggestions if you'd prefer a different approach.
• I'm operating under the assumption that we want to continue to avoid a dependency on any new gems (as we have in the past with the gsl gem). So I'm applying the same patterns that existed before to try loading a library and falling back, which you can see near the top of lsi.rb.
• Note that tests are passing with Numo for ruby 3.1, 3.0, and 2.7! I haven't added any additional tests because I don't expect to change any behavior in an observable way. And I haven't done this yet, but before merging a final version of this PR I plan on building the gem locally and testing it with a real jekyll site to look for any differences in output.

[mattr-]

Took a quick 👀

Looks good so far.

[mattr-]

This looks like it might be debugging code left over. Would you mind removing it?

[mkasberg]

@mattr- This is ready for review! No rush 🙂

I addressed your previous comment, added a little polish, and updated the docs since your last review.

Also, I tested this on a personal jekyll site (i.e. with gem 'classifier-reborn', path: '~/code/classifier-reborn' in my Gemfile) and verified that using Numo procudes the same recommended sites in the Jekyll output as GSL.

[mattr-]

@jekyllbot: merge +minor