Handle dependencies via Pkg extensions

[dkarrasch]

This handles the two dependencies SparseArrays.jl and Statistics.jl as weak dependencies. Indeed, the only reason LinearMaps.jl depended on these was to overload their functions. Otherwise, LinearMaps.jl has always been independent from these two. Moreover, users could not call, say, sparse without loading SparseArrays.jl, so this is a clear case. This brings package load time of pure LinearMaps.jl down to below 20 ms (from about 300 ms, if not 500 ms and more) on current Julia nightly with SparseArrays.jl out of the sysimage).

Resolves #207.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (f522a8f) 99.67% compared to head (98d1014) 99.67%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #208 +/- ## ======================================= Coverage 99.67% 99.67% ======================================= Files 19 21 +2 Lines 1538 1540 +2 ======================================= + Hits 1533 1535 +2 Misses 5 5 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaLinearAlgebra/LinearMaps.jl/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra) | Coverage Δ | | |---|---|---| | [src/LinearMaps.jl](https://app.codecov.io/gh/JuliaLinearAlgebra/LinearMaps.jl/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL0xpbmVhck1hcHMuamw=) | `100.00% <ø> (ø)` | | | [src/conversion.jl](https://app.codecov.io/gh/JuliaLinearAlgebra/LinearMaps.jl/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL2NvbnZlcnNpb24uamw=) | `100.00% <ø> (ø)` | | | [src/linearcombination.jl](https://app.codecov.io/gh/JuliaLinearAlgebra/LinearMaps.jl/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL2xpbmVhcmNvbWJpbmF0aW9uLmps) | `100.00% <ø> (ø)` | | | [src/wrappedmap.jl](https://app.codecov.io/gh/JuliaLinearAlgebra/LinearMaps.jl/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL3dyYXBwZWRtYXAuamw=) | `100.00% <ø> (ø)` | | | [ext/LinearMapsSparseArraysExt.jl](https://app.codecov.io/gh/JuliaLinearAlgebra/LinearMaps.jl/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-ZXh0L0xpbmVhck1hcHNTcGFyc2VBcnJheXNFeHQuamw=) | `100.00% <100.00%> (ø)` | | | [ext/LinearMapsStatisticsExt.jl](https://app.codecov.io/gh/JuliaLinearAlgebra/LinearMaps.jl/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-ZXh0L0xpbmVhck1hcHNTdGF0aXN0aWNzRXh0Lmps) | `100.00% <100.00%> (ø)` | | | [src/blockmap.jl](https://app.codecov.io/gh/JuliaLinearAlgebra/LinearMaps.jl/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaLinearAlgebra#diff-c3JjL2Jsb2NrbWFwLmps) | `100.00% <100.00%> (ø)` | |

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[dkarrasch]

@oschulz This should help your package, shouldn't it? It seems like your package doesn't depend on SparseArrays.jl, so you would avoid loading it, at least via LinearMaps.jl.

[oschulz]

@oschulz This should help your package, shouldn't it?

Yes, definitely! I have this MatrixLikeOperator in there only to keep load time down and to experiment with an operator that subtypes AbstractMatrix. I currently use LinearMaps via an extension - as soon as a `MatrixLikeOperator is multipled with a LinearOperator the result is "promoted" to a LinearOperator. But if LinearOperators becomes even more lightweight I may be able to throw this MatrixLikeOperator out completely.

[oschulz]

Nice, LinearMaps loads in 19 ms now. :-)

[oschulz]

Are you tagging a new relase with this @dkarrasch ?