Add report method for merging fit reports with operational reports

[ablaom]

In support of JuliaAI/MLJBase.jl#831.

A non-breaking enhancement.

In this PR we:

• Add a new model method report(::Model, report_given_method) for combining the reports generated by a model's fit method, with reports generated by operations, such as predict. Here report_given_method is a dictionary of reports, keyed on methods as symbols (:fit, :predict, :transform, and so forth).
• Bump the Julia requirement to 1.6

The docstring for the new method is copied below.

MLJModelInterface.report(model, report_given_method)

Merge the reports in the dictionary report_given_method into a single
property-accessible object. It is supposed that the keys of the dictionary are
:fit and the symbolic names of MLJModelInterface.jl operations, such as :predict or
:transform. Each value will be the report component returned by a training method
(fit or update) dispatched on the model type, in the case of :fit, or the
report component returned by an operation that supports reporting.

New model implementations

Overloading this method is optional, unless the model generates reports that are neither
named tuples nor nothing.
Assuming each dictionary value is a named tuple or nothing, the fallback returns the
usual named tuple merge of the dictionary values, ignoring any nothing values, and
assuming there are no conflicts between the keys of the dictionary values (the individual
reports). If there is a key conflict, all operation reports are first wrapped in a named
tuple of length one, as in (predict=predict_report,).
If any dictionary value is neither a named tuple nor nothing, it is first wrapped as
(report=value, ) before merging.

[codecov-commenter]

Codecov Report

Merging #160 (ed0fd8c) into dev (7cd0ecf) will decrease coverage by 0.05%.
The diff coverage is 84.21%.

@@            Coverage Diff             @@
##              dev     #160      +/-   ##
==========================================
- Coverage   88.75%   88.70%   -0.06%     
==========================================
  Files           8        8              
  Lines         329      354      +25     
==========================================
+ Hits          292      314      +22     
- Misses         37       40       +3     

Impacted Files	Coverage Δ
src/metadata_utils.jl	98.36% <50.00%> (-1.64%)	⬇️
src/model_api.jl	77.77% <88.23%> (+17.77%)	⬆️
src/equality.jl	100.00% <0.00%> (ø)
src/model_def.jl	100.00% <0.00%> (ø)

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[ablaom]

@OkonSamuel

[OkonSamuel]

LGTM. Except for some questions I asked that needs to be addressed.