Move LRP into RelevancePropagation.jl

[adrhill]

Closes #152.

This PR removes ExplainableAI's LRP implementation, which has been moved to RelevancePropagation.jl.
XAI methods that heavily rely on Flux primitives should be added to RelevancePropagation.jl in the future.

This allows us to make ExplainableAI.jl a much more light-weight package with fewer dependencies and should signal the possibility to use the package without Flux. Additionally, this dramatically speeds up package testing.

This builds on work on a common interface in XAIBase.jl (see #154) as well as new common documentation using MultiDocumenter.jl:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (295266c) 95.04% compared to head (ca8d7bd) 89.79%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
- Coverage   95.04%   89.79%   -5.25%     
==========================================
  Files          17        5      -12     
  Lines         706       98     -608     
==========================================
- Hits          671       88     -583     
+ Misses         35       10      -25     

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