CartesianIndex constructor for BandIndex

[jishnub]

This allows easier conversion from a BandIndex to a CartesianIndex by using the CartesianIndex constructor. It also skips the intermediate conversion in to_indices where we need the (row, column) tuple directly.

After this, the following is possible:

julia> b = LinearAlgebra.BandIndex(0, 2)
BandIndex(0, 2)

julia> c = CartesianIndex(b)
CartesianIndex(2, 2)

julia> LinearAlgebra.BandIndex(c)
BandIndex(0, 2)

I had not added the constructor earlier to avoid potential invalidations, but I don't think this is a factor, given that Base already defines four methods for CartesianIndex.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.74%. Comparing base (e7a8a15) to head (0509acb).
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1304   +/-   ##
=======================================
  Coverage   93.73%   93.74%           
=======================================
  Files          34       34           
  Lines       15682    15689    +7     
=======================================
+ Hits        14699    14707    +8     
+ Misses        983      982    -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[dkarrasch]

SGTM