Load and Saving Functionalities for MAP-Elites

[Lookatator]

Add standard save and load methods for the MAPElites class, and the emitter states.

TODOs:

• Tests
• Documentation

[codecov-commenter]

Codecov Report

Merging #91 (14f5e99) into develop (be464f2) will decrease coverage by 0.76%.
The diff coverage is 41.66%.

@@             Coverage Diff             @@
##           develop      #91      +/-   ##
===========================================
- Coverage    89.49%   88.72%   -0.77%     
===========================================
  Files           66       68       +2     
  Lines         3864     3984     +120     
===========================================
+ Hits          3458     3535      +77     
- Misses         406      449      +43     

Impacted Files	Coverage Δ
qdax/utils/serialization.py	35.29% <35.29%> (ø)
qdax/core/map_elites.py	72.41% <38.46%> (-27.59%)	⬇️
qdax/core/emitters/emitter.py	82.14% <66.66%> (-12.31%)	⬇️
qdax/core/neuroevolution/buffers/buffer.py	99.09% <0.00%> (-0.91%)	⬇️
qdax/core/neuroevolution/losses/td3_loss.py	100.00% <0.00%> (ø)
tests/core_test/td3_test.py
tests/core_test/cmamega_test.py
tests/core_test/dads_smerl_test.py
tests/core_test/diayn_smerl_test.py
tests/core_test/pgame_test.py
... and 16 more

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