fix bug in prediction and upgrade nndmd

[pswpswpsw]

In this version, we fixed a bug in koopman.simulate where the model was essentially doing one-step prediction over and over.

We also upgraded the NNDMD regressor so that one can train on list of trajectories even at the koopman level. Do not have to go to the lower regressor level to enjoy the benefits.

[codecov-commenter]

Codecov Report

Merging #43 (24d4ec1) into master (4e38e25) will increase coverage by 0.29%.
The diff coverage is 86.04%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
+ Coverage   72.99%   73.29%   +0.29%     
==========================================
  Files          42       42              
  Lines        3781     3827      +46     
==========================================
+ Hits         2760     2805      +45     
- Misses       1021     1022       +1     

Files	Coverage Δ
src/pykoopman/observables/_time_delay.py	90.56% <ø> (ø)
test/test_koopman.py	99.37% <100.00%> (+0.39%)	⬆️
src/pykoopman/koopman.py	88.35% <91.66%> (+2.11%)	⬆️
src/pykoopman/observables/_identity.py	89.28% <60.00%> (-6.72%)	⬇️
src/pykoopman/common/validation.py	82.35% <33.33%> (-7.65%)	⬇️
src/pykoopman/regression/_nndmd.py	80.53% <75.00%> (-0.38%)	⬇️

... and 6 files with indirect coverage changes