Add support for subclassing in Groups to the QueryBuilder

[sphuber]

Fixes #3902

This now allows to search for instances of Group and all its
subclasses through the QueryBuilder just as implemented for the Node
class. The subclassing is based on the type_string of the group
instances, which in turn is defined by the entry point name. When
querying for Group instances through the QueryBuilder:

QueryBuilder().append(Group).all()

by default subclassing is enabled and should return all instances of a
subclass of Group as well. The same goes for subclasses of Group
themselves. Imagine a plugin defines the following entry points:

'plugin.sub = aiida_plugin.groups:SubGroup'
'plugin.sub.a = aiida_plugin.groups:SubAGroup'
'plugin.sub.b = aiida_plugin.groups:SubBGroup'

The following query:

QueryBuilder().append(SubGroup).all()

will return all instances of SubGroup but also of SubAGroup and
SubBGroup. This is because their entry point names both start with the
entry point name plugin.sub of the class that is being appended.

Note that in order for all group instances to be returned when querying
for the Group base class, a small hack had to be applied in the query
builder code building the filters. Note that the entry point name of
Group is core and so with the rules described above, the plugin
subclasses would never be matched since their entry point names do not
start with core. This is a problem and could be fixed by making the
type string of base group instances an empty string, however, this would
require using an empty string for the Group base class which is not
allowed in Python. To still provide the desired behavior, when querying
for Group instances the type_string filter is set to match anything.

[codecov]

Codecov Report

Merging #3903 into develop will increase coverage by 0.04%.
The diff coverage is 93.93%.

@@             Coverage Diff             @@
##           develop    #3903      +/-   ##
===========================================
+ Coverage    78.08%   78.13%   +0.04%     
===========================================
  Files          458      460       +2     
  Lines        33916    33978      +62     
===========================================
+ Hits         26485    26549      +64     
+ Misses        7431     7429       -2     

Flag	Coverage Δ
#django	70.15% <81.81%> (+0.01%)	⬆️
#sqlalchemy	71.00% <85.45%> (+0.02%)	⬆️

Impacted Files	Coverage Δ
aiida/orm/implementation/groups.py	72.30% <0.00%> (ø)
aiida/plugins/entry_point.py	88.18% <ø> (ø)
aiida/orm/nodes/data/upf.py	80.42% <75.00%> (-0.41%)	⬇️
aiida/tools/groups/paths.py	90.50% <82.35%> (+0.14%)	⬆️
aiida/plugins/factories.py	98.87% <87.50%> (-1.13%)	⬇️
aiida/orm/querybuilder.py	79.90% <94.87%> (+0.47%)	⬆️
aiida/orm/groups.py	91.66% <97.43%> (+4.27%)	⬆️
...s/djsite/db/migrations/0044_dbgroup_type_string.py	100.00% <100.00%> (ø)
aiida/backends/djsite/db/migrations/__init__.py	68.83% <100.00%> (ø)
...tions/versions/bf591f31dd12_dbgroup_type_string.py	100.00% <100.00%> (ø)
... and 13 more

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[chrisjsewell]

Yep that looks good to me