Various interface changes and minor performance improvements

[nicholaskl97]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Changes

Various interface/performance changes, including:

• Added name keyword to NeuralLyapunovPDESystem and removed neural network function output for that function (so now it only outputs a PDESystem)
• Moved NumericalNeuralLyapunovFunctions to its own file and renamed it get_numerical_lyapunov_function; combined methods into one and changed parameter input (now takes res.u.depvar instead of res.u)
• Changed documentation from indicitave to imperative mood
• Changed relu keyword to rectifier
• Changed pendulum parameters in tests to match convention
• Modified tests to improve code coverage (damped_sho.jl now uses ExponentialDecrease and damped_pendulum.jl now uses a softplus-like rectifier)

Additionally, cleaned up local_Lyapunov (now local_lyapunov):

• Renamed
• Added a test
• Removed its $\nabla V$ output
• Changed its $\dot{V}$ calculation
• Split different Jacobian handling into separate methods

[nicholaskl97]

Changes to NumericalNeuralLyapunovFunctions (now get_numerical_lyapunov_function) include closer adherence to best practices for closures, partially addressing #19.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 76.63%. Comparing base (a4d1793) to head (7e1a4c6).
Report is 3 commits behind head on master.

Files	Patch %	Lines
src/numerical_lyapunov_functions.jl	81.48%	5 Missing ⚠️
src/decrease_conditions.jl	60.00%	2 Missing ⚠️
src/NeuralLyapunovPDESystem.jl	75.00%	1 Missing ⚠️
src/minimization_conditions.jl	0.00%	1 Missing ⚠️
src/policy_search.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #24       +/-   ##
===========================================
+ Coverage   66.25%   76.63%   +10.38%     
===========================================
  Files           9       10        +1     
  Lines         243      244        +1     
===========================================
+ Hits          161      187       +26     
+ Misses         82       57       -25     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.