Deprecate internal code loading; add runtime inclusion option.

[willow-ahrens]

This change would deprecate the custom code loading in MatrixDepot. In particular, this makes three key changes:

1. MatrixDepot will no longer create or write to MYDEPOT. While MatrixDepot still reads these files, it will issue a deprecation warning if they are not the default files.
2. MatrixDepot no longer serializes user-defined generators because they may or may not be loaded when the database is deserialized
3. The Documentation now explains the replacement functionality, which directs users to call include_generator from outside of MatrixDepot at runtime, then reinitialize.

[willow-ahrens]

Note that we cannot remove the code associated with determining the path of MYDEPOT until we move from deprecation to removal at some point in the future.

[KlausC]

This PR is targeting only your clone of https://github.com/JuliaMatrices/MatrixDepot.jl, what explains, that we did not receive notifications.

[willow-ahrens]

That's correct, this is a PR on a PR (JuliaLinearAlgebra#75). If you'd prefer, I can retarget this as a PR directly to MatrixDepot after JuliaLinearAlgebra#75 is merged.

[willow-ahrens]

These changes have been moved to JuliaLinearAlgebra#79