Overload isempty method for types.untyped.Set (Issue #561)

[ehennestad]

Fix #561

Motivation

See issue #561

How to test the behavior?

>> emptySet = types.untyped.Set();
>> isempty(emptySet)

ans =

  logical

   1

Checklist

• Have you ensured the PR description clearly describes the problem and solutions?
• Have you checked to ensure that there aren't other open or previously closed Pull Requests for the same change?
• If this PR fixes an issue, is the first line of the PR description fix #XX where XX is the issue number?

[ehennestad]

The following snippet causes a lot of tests to fail, but is not relevant anymore:

matnwb/+io/parseGroup.m

Lines 66 to 70 in 3222f33

	if isempty(parsed)
	%special case where a directory is simply empty. Return itself but
	%empty
	parsed(root) = [];
	end

The code was embedded in this context:

    %immediately elide prefix all property names with this but only if there are
    %no typed objects in it.
    propnames = keys(props);
    if hasTypes
        parsed = containers.Map({root}, {props});
    else
        parsed = containers.Map;
        for i=1:length(propnames)
            pnm = propnames{i};
            p = props(pnm);
            parsed([root '_' pnm]) = p;
        end
    end
    
    if isempty(parsed)
        %special case where a directory is simply empty.  Return itself but
        %empty
        parsed(root) = [];
    end

It was possible to create a containers.Map object named parsed that was empty, but parsed was later changed (see below) to be a types.untyped.Set which per definition (ref this PR) can not be empty (unless specifically created with types.untyped.Set.empty)

matnwb/+io/parseGroup.m

Lines 56 to 63 in 5aa27d9

	parsed = types.untyped.Set(...
	[attributeProperties; datasetProperties; groupProperties; linkProperties]);
	if isempty(parsed)
	%special case where a directory is simply empty. Return itself but
	%empty
	parsed(root) = [];
	end

Code coverage also shows that lines are not covered by tests. Will remove this part

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.95%. Comparing base (5aa27d9) to head (6bcafd2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #616      +/-   ##
==========================================
+ Coverage   90.91%   90.95%   +0.04%     
==========================================
  Files         107      107              
  Lines        4753     4752       -1     
==========================================
+ Hits         4321     4322       +1     
+ Misses        432      430       -2     

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[ehennestad]

Current thinking: This should be not be merged as is. A types.untyped.Set is similar to a struct, containers.Map or dictionary in that it holds name-value pairs. Non of the builtin MATLAB types are empty if they have no name-value pairs, i.e s = struct is a scalar, and s = struct.empty is empty.

The same should be true for the Set.

Instead of redefining is empty, I think it would be better to not override size as is currently done, and also to change the custom display for a Set with no elements from:
Empty Set
to:
Set with no elements or Set with no name, value pairs