Make bsplines! return OffsetArray

[sostock]

This makes the output of bsplines!(dest, args...) equivalent to that of bsplines(args...). The returned OffsetArray is a wrapper around dest.

[codecov-commenter]

Codecov Report

Merging #8 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master       #8   +/-   ##
=======================================
  Coverage   89.95%   89.95%           
=======================================
  Files           6        6           
  Lines         428      428           
=======================================
  Hits          385      385           
  Misses         43       43           

Impacted Files	Coverage Δ
src/bsplinebasis.jl	97.32% <100.00%> (ø)

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