[develop < T0041-GA] Fix label filtering in where queries

[katarinasupe]

In this PR, I wanted to check how to create private properties which won't be saved in the database and how to load a node with partial properties, that is, how to load a node with only required properties provided.

That is related to issues #88 and #79.

Conclusions:

• A property without Field() will be saved into Memgraph. If you don't want a property to be saved into Memgraph, declare it as private property (e.g. _name). All properties can be used without Field() and schema validation will still work. Field must be used when defining additional fields, such as constraints and indexes.

• I think it's okay that load works the way it does. If we want to fetch nodes by one property, we can use query builder, that is match().

While working on this, I noticed a couple of more things I added to this PR:

• WHERE clause and all its versions (and, or, xor) had label filtering added, but this was not tested. I added tests and fixed methods in the query builder accordingly.

• Changed property to item in xor_where() arguments

• Fixed WHERE methods in query builder (where, and where, or where, xor where) to work with labels

• Added tests for label filtering in WHERE methods (where, and where, or where, xor where)

• Added tests for node loading and properties usage

[katarinasupe]

Please ignore where() method in query builder here. We need to merge this before where() PR. This PR is just couple of new tests in the end.

[BorisTasevski]

Classes as fixtures should also be discussed here