Move BandedMatrices+BlockArrays to BlockArray extension

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 89.53%. Comparing base (1425899) to head (33fe6b9). Report is 1 commits behind head on release-0.13.

Files	Patch %	Lines
src/BandedBlockBandedMatrix.jl	88.88%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release-0.13 #195 +/- ## ================================================ + Coverage 88.37% 89.53% +1.15% ================================================ Files 11 11 Lines 1204 1108 -96 ================================================ - Hits 1064 992 -72 + Misses 140 116 -24 ```

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[jishnub]

Let's create a release-0.13 branch, as this would simplify collecting all the changes.

Edit: I've created one now. Would you mind changing the target branch for this PR?

[dlfivefifty]

Done. But what else is going in release-0.13?

[jishnub]

There are some other minor changes that might be useful, like not exporting Zeros/Ones/Fill/Block and asking users to import these from the corresponding packages.

[dlfivefifty]

What's wrong with exporting them?

[jishnub]

It just makes the package more prone to breakages, as breaking changes upstream get translated to this package as well, which shouldn't be the case usually. Typically, one may bump the minor or patch version of a package to incorporate a breaking version of a dependency.

[dlfivefifty]

ok I see your point