Add invalidate_cache function to Memory public interface

[khemarato]

Adds a simple function to the public interface for a Memory-ized function that allows the user to clear just one value from the cache. Luckily, the store_backend already has the functionality to do this, it just needs to be exposed.

[fcharras]

Fixes #1559

[fcharras]

@buddhist-uni thank you for the PR.

Before merging it requires adding at least a test to Memory test suite at joblib/test/test_memory.py, also a usage example in examples/memory_basic_usage.py would be a nice addition.

Would you like to continue pushing this PR forward or should we complete from your initial commit ?

[khemarato]

Please take it from here 😊