Take part of a matrix by range (#443)

[Rafael-SOWNet]

Closes #443.

var a = MathS.Matrices.Matrix(3, 3,
    1, 2, 3,
    4, 5, 6,
    7, 8, 9);

a[1.., ..]     // 4 5 6 / 7 8 9
a[.., ..2]     // 1 2 / 4 5 / 7 8
a[1..3, 1..3]  // 5 6 / 8 9
a[^1.., ^1..]  // 9

The result is an ordinary matrix sharing nothing with the one it came from, so it transposes, multiplies and compares like any other.

Both extents have to be written out

@Happypig375 asked on the issue for a[1.., ..] rather than a[1..], and that is what this does — there is no one-range indexer, so the shorter form does not compile.

The reason it would be ambiguous is already in the type: this[int i] gives the i-th row of a matrix but the i-th element of a vector, since a vector is a matrix of one column. A single range would have to inherit that and mean one thing for one shape and another for the other. With both extents written out, a vector is cut by its first extent like anything else and Column[1.., ..] drops its first element.

Edges

An empty range is refused with InvalidMatrixOperationException rather than answered with a matrix of no rows, because there is no such matrix. A range reaching past the matrix throws ArgumentOutOfRangeException from Range.GetOffsetAndLength itself, so the message names the real problem.

System.Range compiles on both net7.0 and netstandard2.0 here, so the API is the same on both — I checked before writing it rather than assuming.

Verification

Ten tests covering each form above, the whole-matrix case, a vector, symbolic entries carried over untouched, and both refusals. 3890 unit tests and 127 F# tests on both target frameworks, none failing.

These tests do not fail before the change — they do not compile, since the indexer they call does not exist. That is the only "before" a new API has.

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✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.14%. Comparing base (90c00a8) to head (302c48c).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #683      +/-   ##
==========================================
- Coverage   80.99%   80.14%   -0.85%     
==========================================
  Files         155      156       +1     
  Lines       13687    12648    -1039     
  Branches     1957     2064     +107     
==========================================
- Hits        11086    10137     -949     
+ Misses       1990     1914      -76     
+ Partials      611      597      -14     

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