Enhancements for Assemblies

As discussed in https://github.com/bernhard-42/jupyter-cadquery/issues/23 I had added some enhancements to Assemblies which might be helpful for cadquery:

## 1 Enhancements
### 1.1 Chained assembly selectors "name1>name2>name3"

**Problem:** Using the same assemblies several times in an assembly

Simplified example [full code](https://github.com/bernhard-42/jupyter-cadquery/blob/master/examples/assemblies/2-hexapod.ipynb):

```python
# Assembly 1: The leg
leg = Assembly(upper_leg, name="upper").add(lower_leg, name="lower")

# Assembly 2: The six legged Hexapod
hexapod = (Assembly(base, name="bottom"))
for name in ['right_back', 'right_middle', 'right_front', 'left_back', 'left_middle', 'left_front']:
hexapod.add(leg, name=name)
```

Now `hexapod.objects` only has one `lower` element:

```
{'bottom': ,
'right_back': ,
'upper': ,
'lower': ,
'right_middle': ,
'right_front': ,
'left_back': ,
'left_middle': ,
'left_front': }
```

If you need to select the `lower` leg of the left back leg (e.g. to assemble yet another object), it doesn't seem to be obvious.

**Solution approach:** I enhanced the assembly selector to support the chain `bottom>right_back>lower` or, because it's also unique, `right_back>lower`. In `MAssembly` this is done via [this function](https://github.com/bernhard-42/jupyter-cadquery/blob/e14f316dd9384f0586e21c5fdfa46f1d8931f697/jupyter_cadquery/mate_assembly/massembly.py#L53-L75)

### 1.2 Chaining object selectors

**Problem**: Many times one selects components via a chain, e.g. `cad_obj.faces(">Z").edges(">X")`. I did not see a way to do this via `"obj_name?tag1@faces@>Z"` query

**Solution Approach**: Besides string queries also allow tuple queries like `("obj_name?tag", "faces@>Z")` and allow more than one object selector, e.g.: `("obj_name?tag", "faces@>Z", "edges@>Y")`.
Note: `"obj_name?tag1@faces@>Z"` and `("obj_name?tag", "faces@>Z")` lead to the same result.

Of course, one could also use e.g. `obj_name?tag1@faces@>Z@@edges@>X`, but for me this seems to be harder to read and I anyhow needed the tuple selector for problem 3

### 1.3 Selecting one of several holes on a face

**Problem**: If you have e.g. several holes on a face and want to select a specific one, the only way that came to my mind was to use the `NearestToPointSelector`. But I wanted to use that with a simple query again.

**Solution approach:** Given the tuple query from above, I defined a new pattern: `("wires", (x,y))`. This allowed me to do

```python
for coord_2d in hole_coords:
sel = ("obj_name?tag", "faces@>Z", ("wires", coord_2d))
...
```

In `MAssembly` 1.2 and 1.3 are implemented with [these functions](https://github.com/bernhard-42/jupyter-cadquery/blob/e14f316dd9384f0586e21c5fdfa46f1d8931f697/jupyter_cadquery/mate_assembly/massembly.py#L77-L138), however, currently without tag support.

## 2 Questions

1) What is your take on my 3 problems. Do you see easier solutions already with the existing Assembly selectors?

2) Would you be interested in a PR to solve these three problems (assuming there is no simple solution at the moment)

3) The classes `MAssembly` and `Mate` are a little bit of aliens in my CAD viewer repo. Since the `Massembly` approach is based on `cq.Assembly` would you be interested in a PR to add the property `mates` and the methods [mate()](https://github.com/bernhard-42/jupyter-cadquery/blob/e14f316dd9384f0586e21c5fdfa46f1d8931f697/jupyter_cadquery/mate_assembly/massembly.py#L140-L155), [assemble()](https://github.com/bernhard-42/jupyter-cadquery/blob/e14f316dd9384f0586e21c5fdfa46f1d8931f697/jupyter_cadquery/mate_assembly/massembly.py#L177-L190) to `cq.Assemblies` (both methods have 8 lines of code each) and of course the [Mate](https://github.com/bernhard-42/jupyter-cadquery/blob/e14f316dd9384f0586e21c5fdfa46f1d8931f697/jupyter_cadquery/mate_assembly/mate.py) class?
This would add a static assembly approach (similar to cqparts and Assembly4 of FreeCad) parallel to your dynamic solver.

4) If you would add the `MAssembly` functionality to `cq.Assembly`, would it be OK to support the `origin` property that holds the origin mate of an Assembly, even if this is only necessary for jupyter_cadquery? Otherwise I would need to monkey-patch CadQuery for origin support.
The `relocate()` functions could be omitted, since they are only necessary for the animations system in jupyter-cadquery.

These are just a few ideas being developed while learning about assemblies and I would be happy to contribute some stuff to CadQuery if you are interested ...