Add MinkowskiMetric abstract type

[eliascarv]

This new type will allow third-party packages to differentiate Minkowski metrics at the type level. It will also allow third-party packages to implement their own Minkowski metrics.

A practical example of a package that could use this new abstract type is NearestNeighbors.jl, which is currently using a union type for this purpose:

https://github.com/KristofferC/NearestNeighbors.jl/blob/master/src/kd_tree.jl#L1

With this new type NearestNeighbors.j could use Minkowski metrics defined in any package.

[juliohm]

Appreciate if a maintainer can review and merge this.

[juliohm]

ping @KristofferC @dkarrasch

[dkarrasch]

This doesn't seem to hurt anyone, so this LGTM, except for the consideration of adding two types vertically into the type hierarchy.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.50%. Comparing base (886ad02) to head (c9587a1).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #265      +/-   ##
==========================================
+ Coverage   95.86%   96.50%   +0.64%     
==========================================
  Files          10       10              
  Lines         944      888      -56     
==========================================
- Hits          905      857      -48     
+ Misses         39       31       -8     

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[dkarrasch]

Looks much cleaner now and non-breaking.

[dkarrasch]

LGTM

[eliascarv]

@nalimilan, this PR is ready. Can you review it please?