Added ASCII representations for orbitals and configurations

[jagot]

Still need to fix printing of spin-configurations after the unsorted debacle and introduce ASCII representations of those.

[codecov[bot]]

Codecov Report

Merging #55 into master will increase coverage by 0.11%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   91.35%   91.47%   +0.11%     
==========================================
  Files          15       15              
  Lines         810      821      +11     
==========================================
+ Hits          740      751      +11     
  Misses         70       70

Impacted Files	Coverage Δ
src/configurations.jl	98.69% <100%> (+0.03%)	:arrow_up:
src/orbitals.jl	94.59% <100%> (+0.2%)	:arrow_up:

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[jagot]

What is a nice ASCII representation for spin up/down? These would be used for printing, but also for construction of spin-orbitals using string macros.

Ideas

Spin-orbitals always have degeneracy unity, so we never need to juxtapose a occupation number after the orbital string.

Non-relativistic spin-orbitals

For non-relativistic spin-orbitals we have the quantum numbers n, ell, m_ell, s=1/2, and m_s.

Spin up	Spin down	2p w/ m_ell=0 & spin up	Notes
^	v	2p0^
+	-	2p0+
a	b	2p0a	Consistent with α & β used in Unicode representation

Relativistic spin-orbitals

For relativistic orbitals, we instead have the quantum numbers n, ell, j=ell±1/2, and m_j. We already denote j = ell-1/2 by minus (-) and j = ell + 1/2 by silence. How to tack m_j onto this?

I think the least ambiguous is n ell [-] (m_j), e.g.

• 2p-(-1/2)
• 2p(-1/2) (without the parentheses, the minus could be inferred to mean j = ell - 1/2.

[jagot]

Superseded by #57