Cleanup FIML, enhance em_mvn()

[alyst]

This is hopefully one of the 2 big remaining refactorings from #193:

• FIML: introduce SemFIMLPattern -- it handles FIML calculation for a specific missing pattern.
  Previously, we introduced SemObservedMissingPattern to collect observed samples with a specific missing pattern; this PR applies a similar change for FIML.
• em_mvn(): decouple from SemObservedMissing. Previously one could only use em_mvn() from SemObservedMissing after it's already created.
  Now it could be used outside, and there is no EmMVNModel struct.
  obs_cov and obs_mean for SemObservedMissing are still lazily calculated, but now it follows the common SemObserved API, so there is less
  need for overloading methods to handle SemObservedMissing
• em_mvn() was optimized:
  • do more inplace linear algebra to reduce memory footprint
  • use Cholesky factorizations of the matrices (obs cov within a missing pattern) instead of matrices themselves.
  • more subtle numerical stability optimizations to work with z (observed data of the current pattern x minus the current mean estimate) rather than with x directly.
• em_mvn() some options to help with large matrices:
  • max_nsamples_em limits how many samples from each missing pattern is used -- to handle data with thousands of samples
  • min_eigval to enforce pos-def of the resulting matrix
• add progress bar support to em_mvn() for monitoring the convergence of large matrices (thousands of samples, >1000 observed vars)

[alyst]

cc @Maximilian-Stefan-Ernst

[codecov]

Codecov Report

❌ Patch coverage is 85.02415% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.45%. Comparing base (1f8d2a9) to head (4989950).
⚠️ Report is 28 commits behind head on devel.

Files with missing lines	Patch %	Lines
src/additional_functions/helper.jl	0.00%	16 Missing ⚠️
src/observed/EM.jl	90.82%	10 Missing ⚠️
src/loss/ML/FIML.jl	94.73%	3 Missing ⚠️
src/observed/missing.jl	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel     #303      +/-   ##
==========================================
- Coverage   71.83%   71.45%   -0.39%     
==========================================
  Files          51       51              
  Lines        2223     2288      +65     
==========================================
+ Hits         1597     1635      +38     
- Misses        626      653      +27     

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[Maximilian-Stefan-Ernst]

Looks nice, also the imporvements to the EM algorithm. I made some small comments, apart from that, I'm happy to merge.

[alyst]

@Maximilian-Stefan-Ernst thank you for the review! I've addressed your comments in the recent commits (If you approve the PR, I will cleanup the history before the merge).
I've also rebased the branch to the latest devel branch. The CI test is passing, the format checks fail -- but in the fiels that were not touched by this PR.

[Maximilian-Stefan-Ernst]

Thank you, I would be happy to merge now - do you want to clean anything up still, or should I go ahead?

[alyst]

@Maximilian-Stefan-Ernst I've just rebased it to the devel branch and cleaned up the commit history. It should be ready to merge. Thank you!