[WIP] Allow more general knot sequences

[sostock]

This PR attempts to implement support for knot sequences where the first and last breakpoints appear less than order(basis) times. It is currently work in progress, and at a very early stage.

• Store knots instead of breakpoints in BSplineBasis
• Add a constructor with keyword arguments to be able to specify the knot vector instead of the breakpoints: BSplineBasis(k, knots=...) and BSplineBasis(k, breakpoints=...). Deprecate the old (non-keyword) constructor.
• Allow indexing into a BSplineBasis with a UnitRange to create a new BSplineBasis. Support view as well.
• Make bsplines/bsplines! return an OffsetArray of a view.
• Modify bsplines/bsplines!/splinevalue to work with the new knot vectors.
• Deprecate indices keyword argument, recommend to use view(::BSplineBasis, ...) instead.
• Update documentation

[codecov-io]

Codecov Report

Merging #5 (633ac00) into master (342976f) will increase coverage by 0.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   95.20%   95.50%   +0.29%     
==========================================
  Files           6        6              
  Lines         501      534      +33     
==========================================
+ Hits          477      510      +33     
  Misses         24       24              

Impacted Files	Coverage Δ
src/plotting.jl	34.37% <ø> (ø)
src/BSplines.jl	100.00% <100.00%> (ø)
src/bsplinebasis.jl	99.67% <100.00%> (+0.03%)	⬆️
src/spline.jl	97.61% <100.00%> (ø)

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