BREAKING: Replace alias functions with constant object bindings

[jmert]

• Generalize the callable object functions to act on all AbstractLegendreNorm, not just LegendreNormCoeff (on Julia v1.3+).
• Replace Plm[!] and λlm[!] functions with constant bindings to the corresponding UnitLegendreNorm() and LegendreSphereNorm() singletons.
• Remove the Pl function to avoid future ambiguity of calls like

  Pl!(out, l, in)
  Plm(l, m, in)

  where the 3-argument functions are ambiguous if the type constraints need to be relaxed.

[codecov[bot]]

Codecov Report

Merging #3 into master will decrease coverage by 0.89%. The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master       #3      +/-   ##
==========================================
- Coverage   98.07%   97.17%   -0.90%     
==========================================
  Files          10        9       -1     
  Lines         260      248      -12     
==========================================
- Hits          255      241      -14     
- Misses          5        7       +2     

Impacted Files	Coverage Δ
src/norm_table.jl	93.54% <ø> (-1.05%)	:arrow_down:
src/calculation.jl	96.85% <66.66%> (-1.50%)	:arrow_down:
src/broadcasting.jl	100.00% <100.00%> (ø)
src/precompile.jl	100.00% <100.00%> (ø)

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