Ensure matmul in periodic bspline fit dispatches to specialized sparse method when ys is a Vector eltype

[halleysfifthinc]

While profiling some of my code that fits a periodic b-spline on multi-dimensional points, I
noticed a generic matmul taking a substantial amount of the fit runtime (~30%).

Some @descending later, I determined that the specialized sparse matmul method signature
requires the destination matrix to be a <:StridedMatrix, and a transposed
reshape/reinterpreted matrix isn't.

I tweaked the reinterpret/reshapeing to keep the destination array <:StridedMatrix when
ys are multi-dimensional points (e.g. SVector).

master:

julia> @benchmark fit(BSplineOrder(4), $ts, vvs, λ, Periodic(2π)) setup=(vvs=copy(vs))
BenchmarkTools.Trial: 10000 samples with 1 evaluation per sample.
 Range (min … max):  170.360 μs …   6.467 ms  ┊ GC (min … max): 0.00% … 83.29%
 Time  (median):     200.202 μs               ┊ GC (median):    0.00%
 Time  (mean ± σ):   215.333 μs ± 134.002 μs  ┊ GC (mean ± σ):  4.43% ±  7.07%

  ▄▇█▅▂▂▁▂▂                                                     ▂
  █████████▇▅▆▃▁▁▃▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▃▃▅▆ █
  170 μs        Histogram: log(frequency) by time          1 ms <

 Memory estimate: 253.10 KiB, allocs estimate: 204.

PR:

julia> @benchmark fit(BSplineOrder(4), $ts, vvs, λ, Periodic(2π)) setup=(vvs=copy(vs))
BenchmarkTools.Trial: 10000 samples with 1 evaluation per sample.
 Range (min … max):  121.648 μs …   6.925 ms  ┊ GC (min … max): 0.00% … 76.07%
 Time  (median):     126.277 μs               ┊ GC (median):    0.00%
 Time  (mean ± σ):   143.236 μs ± 180.047 μs  ┊ GC (mean ± σ):  8.12% ±  6.54%

  ▁▆██▆▄▅▅▄▃▃▂▁                                                 ▂
  █████████████████▇▆▆▅▅▆▅▄▄▄▃▃▅▅▄▄▄▄▃▃▃▁▁▁▃▁▃▁▁▁▁▄▃▄▅▆▇██▇▆▆▇▇ █
  122 μs        Histogram: log(frequency) by time        208 μs <

 Memory estimate: 253.10 KiB, allocs estimate: 204.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.17%. Comparing base (d38bb55) to head (3815c07).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #114   +/-   ##
=======================================
  Coverage   95.17%   95.17%           
=======================================
  Files          36       36           
  Lines        2342     2344    +2     
=======================================
+ Hits         2229     2231    +2     
  Misses        113      113           

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[jipolanco]

This is great, thank you!