Add models for rates and tradeable assets

[sschlenkrich]

This PR adds models for interest rates and tradeable assets:

• multi-factor Gaussian HJM model for rates,
• lognormal model for FX, Equity, indices
• update tests and documentation

[codecov-commenter]

Codecov Report

Merging #6 (d1dd05a) into main (bc4d21b) will decrease coverage by 1.78%.
The diff coverage is 91.84%.

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
- Coverage   95.42%   93.65%   -1.78%     
==========================================
  Files           7       13       +6     
  Lines         175      457     +282     
==========================================
+ Hits          167      428     +261     
- Misses          8       29      +21     

Impacted Files	Coverage Δ
src/DiffFusion.jl	100.00% <ø> (ø)
src/models/Model.jl	80.95% <80.95%> (ø)
src/models/asset/LognormalAssetModel.jl	94.28% <94.28%> (ø)
src/utils/Integrations.jl	94.44% <94.44%> (ø)
src/models/rates/GaussianHjmModel.jl	94.93% <94.93%> (ø)
src/models/asset/AssetModel.jl	100.00% <100.00%> (ø)
src/models/rates/SeparableHjmModel.jl	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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