Allow for BSpline type construction via chained pipes

[mkitti]

The objective of this pull request is to simplify the API. This is accomplished by 1) Providing default constructors. For example, BSpline(), Linear(), Cubic(), Quadratic() 2) Allow for chaining via pipes.

julia> BSpline(Cubic(Line(OnGrid())))
BSpline(Cubic(Line(OnGrid())))

julia> OnGrid |> Line |> Cubic |> BSpline
BSpline(Cubic(Line(OnGrid())))

julia> Line |> Cubic |> BSpline
BSpline(Cubic(Line()))

julia> Cubic |> BSpline
BSpline(Cubic(Line(OnGrid())))

julia> BSpline()
BSpline(Linear())

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (f432a7b) 87.74% compared to head (8fc0764) 87.99%. Report is 24 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #536 +/- ## ========================================== + Coverage 87.74% 87.99% +0.24% ========================================== Files 28 28 Lines 1861 1899 +38 ========================================== + Hits 1633 1671 +38 Misses 228 228 ```

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