Recursive evaluation of derivatives

[jishnub]

This PR introduces the extra functions dnPl, collectdnPl and collectdnPl! that lets us compute the n-th derivative of Legendre polynomials:

julia> dnPl(0.5, 10, 3) # 3rd derivative of P10(x) at x = 0.5
399.04541015625

julia> collectdnPl(0.5, lmax = 10, n = 3) # all 3rd derivatives of Pl(x) for l in 0:lmax
11-element OffsetArray(::Array{Float64,1}, 0:10) with eltype Float64 with indices 0:10:
    0.0
    0.0
    0.0
   15.0
   52.5
   65.625
  -19.6875
 -179.6484375
 -230.09765625
    3.3837890625
  399.04541015625

[codecov]

Codecov Report

Merging #5 (8026ad5) into master (7d70c32) will increase coverage by 2.32%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master        #5      +/-   ##
===========================================
+ Coverage   97.67%   100.00%   +2.32%     
===========================================
  Files           1         1              
  Lines          43       100      +57     
===========================================
+ Hits           42       100      +58     
+ Misses          1         0       -1     

Impacted Files	Coverage Δ
src/LegendrePolynomials.jl	100.00% <100.00%> (+2.32%)	⬆️

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