Improve handling of isometric expansion when collapsing sparsity.

[LTLA]

There are several decisions that need to be made depending on:

1. Whether the operation preserves sparsity.
2. If it doesn't preserve sparsity, whether the zero operation is constant.
3. Whether the underlying matrix is sparse.
4. Whether a sparse extraction was requested.

The operation always needs to define dense() methods for a pointer with block and index extraction.

If either of 1 or 2 is true, the operation should define a dense() method that takes a SparseRange, and a sparse() method that takes a SparseRange.

If 3 is false, we call the dense() methods for a pointer with block and index extraction.

If 3 is true and 1 is true, we call the sparse() method (even if dense output was requested).

If 3 is true and 1 is false and 2 is true, we call dense() on the SparseRange.

If 3 is true and 1 is false and 2 is false, we call dense() on the pointers.

[codecov-commenter]

Codecov Report

Merging #62 (dd4fb4a) into master (1b56d67) will decrease coverage by 0.12%.
The diff coverage is 91.30%.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
- Coverage   95.11%   94.99%   -0.12%     
==========================================
  Files         102      102              
  Lines        9230     9318      +88     
  Branches      155      160       +5     
==========================================
+ Hits         8779     8852      +73     
- Misses        369      379      +10     
- Partials       82       87       +5     

Impacted Files	Coverage Δ
include/tatami/isometric/arith_utils.hpp	100.00% <ø> (ø)
include/tatami/isometric/unary/math_helpers.hpp	95.91% <82.60%> (-4.09%)	⬇️
...tatami/isometric/unary/DelayedUnaryIsometricOp.hpp	94.44% <92.66%> (-1.91%)	⬇️
include/tatami/isometric/unary/arith_helpers.hpp	98.14% <100.00%> (+0.14%)	⬆️
include/tatami/isometric/unary/boolean_helpers.hpp	100.00% <100.00%> (ø)
include/tatami/isometric/unary/compare_helpers.hpp	100.00% <100.00%> (ø)
tests/src/isometric/unary/math_helpers.cpp	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes