Allow block multiplication of non-square blocks

[cvsvensson]

Currently, BlockDiagonals keeps the block structure in matrix multiplication only if the blocks that are multiplied together have the same size. This is overly restrictive and rules out multiplication with non-square blocks.

This PR modifies the check done when multiplying BlockDiagonals to permit multiplication with non-square blocks while retaining the block diagonal structure.

In addition, the overhead of checking if blocks can be multiplied or added has been reduced.

[codecov[bot]]

Codecov Report

Merging #127 (db43d04) into master (00fab43) will increase coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   98.89%   98.93%   +0.04%     
==========================================
  Files           5        5              
  Lines         361      375      +14     
==========================================
+ Hits          357      371      +14     
  Misses          4        4              

Impacted Files	Coverage Δ
src/base_maths.jl	100.00% <100.00%> (ø)
src/linalg.jl	95.91% <100.00%> (-0.09%)	:arrow_down:

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[nathanaelbosch]

What is the status of this PR? Can we get this done and merged?

[jishnub]

I think development will be easier if support for old versions of Julia is dropped. I'll create a PR for that, following which we may rebase this PR and check.