Overhaul handling of array arguments

[jmert]

This does a large overhaul of how array arguments are handled, namely making legendre() a vector function and "downgrading" the status/importance of broadcasting.

Highlights:

• legendre(norm, 0:lmax, 0:mmax, x) will now allocate and return an appropriately-shaped output for calculating all degrees/orders for all x (from scalar through any-dimensional array). This effectively fixes #8.
• Since legendre() now does array-based operations, the advertisement of broadcasted legendre.() has been removed from the manual. There are still broadcasted(::typeof(legendre)) specializations, though, to intercept any accidental broadcasting that would be better served by a single vectorized invocation.
• Renamed the internal _legendre! function to unsafe_legendre!, and moved around argument checking so that it's truly unsafe and no need for @propagate_inbounds (so should save time/effort in compiling multiple long inlining of the full implementation).

[jmert]

Failures on v1.5 and master are due to a change in how LLVM is optimizing the coeff_α and coeff_β functions for the spherical normalization case. The issue has been unnoticed thus far because there were no written/tested examples of particular affected numerical values until the rewrite of the documentation introduced in this PR.

I've asked a generic question on Julia's Discourse to see if there's more information about what LLVM is doing specifically, but in the mean time I'll add an appropriate workaround commit to bring the calculation back into alignment on all Julia versions.

[codecov[bot]]

Codecov Report

Merging #19 into master will increase coverage by 0.27%. The diff coverage is 97.87%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
+ Coverage   95.52%   95.80%   +0.27%     
==========================================
  Files           9        9              
  Lines         246      262      +16     
==========================================
+ Hits          235      251      +16     
  Misses         11       11              

Impacted Files	Coverage Δ
src/calculation.jl	96.15% <95.65%> (+0.42%)	:arrow_up:
src/broadcasting.jl	100.00% <100.00%> (ø)
src/norm_sphere.jl	100.00% <100.00%> (ø)
src/precompile.jl	100.00% <100.00%> (ø)

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