Fix Basis compatibility with ODEProblem

[ChrisRackauckas-Claude]

Summary

This PR fixes issue #552 where using a Basis object obtained from DMD or SINDy as the ODE function in ODEProblem would throw an error about the 'observed' field not existing.

Changes

• Added ModelingToolkit interface methods for Basis in new file src/basis/modelingtoolkit_interface.jl
• Implemented required AbstractSystem interface methods:
  • equations
  • unknowns
  • parameters
  • get_observed
  • get_iv
  • nameof
• Override getproperty to properly handle :observed field access
• Override show method to fix display error related to namespacing flag
• Added conditional definitions for namespacing-related methods to maintain compatibility across ModelingToolkit versions

Test plan

[x] Tested with the MRE from issue #552
[x] All existing package tests pass
[x] Both reported issues (ODEProblem creation and display) are fixed

Fixes #552

🤖 Generated with Claude Code

[ChrisRackauckas-Claude]

Updated per feedback:

• Moved interface methods directly to src/basis/type.jl instead of creating a separate file
• Kept only the necessary overrides:
  • ModelingToolkit interface methods (equations, unknowns, parameters, get_observed, get_iv, nameof)
  • getproperty override for :observed field (required because ModelingToolkit's getproperty for AbstractSystem tries to use getvar which doesn't work for Basis)
  • show override to fix display issue

The getproperty override is necessary because SciMLBase tries to access sys.observed directly, which goes through ModelingToolkit's getproperty implementation that expects getvar to work.