Hashing and reduction of configurations

[mortenpi]

• The hash(::Configuration) makes sure that "isequal(x,y) therefore hash(x) == hash(y)" holds, which is important for unique to work properly with lists of configurations.
• Defines the properties of orbitals that are part of the public API. Adds a computed .ℓ and .j property to RelativisticOrbital, since these are often used.
• Adds a function to reduce relativistic configurations back down to the corresponding non-relativistic orbitals by combining the nℓ- and nℓ shells.

[codecov[bot]]

Codecov Report

Merging #37 into master will increase coverage by 0.35%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #37      +/-   ##
=========================================
+ Coverage   90.35%   90.7%   +0.35%     
=========================================
  Files          15      15              
  Lines         736     753      +17     
=========================================
+ Hits          665     683      +18     
+ Misses         71      70       -1

Impacted Files	Coverage Δ
src/configurations.jl	99.28% <100%> (+0.03%)	:arrow_up:
src/orbitals.jl	94.33% <100%> (+0.22%)	:arrow_up:
src/AtomicLevels.jl	80% <0%> (+20%)	:arrow_up:

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[mortenpi]

Fixed everything other than the MQ naming. Good to go or would you prefer to change it?

[jagot]

No, we can do that later