Functions silently overwritten in Instrument

### Steps to reproduce

```python
from qcodes.instrument.base import Instrument
from qcodes.instrument.function import Function

class MyInst(Instrument):

def __init__(self, name):
super().__init__(name)

self.add_parameter('start', get_cmd="I'm a parameter")
self.add_function('start', call_cmd="I'm a function")

def write(self, cmd):
print(cmd)

def ask(self, cmd):
print(cmd)

myinst = MyInst('myinst')

myinst.start()
```

### Expected behaviour
Since both functions and parameters get delegated as attributes in the same way, it should not be possible to add a parameter with the name of an existing function of vice versa.

### Actual behaviour
The function gets ignored in favour of the parameter. I found a comment indicating that this is by design https://github.com/QCoDeS/Qcodes/blob/cfc97ec2090ed3620634affa3177419dddb6abae/qcodes/tests/test_instrument.py#L75. I don't think this is such a good design decision.

### Proposed solution

Maybe just get rid of the functions altogether?

### System
**operating system**
Windows 10

**qcodes branch**
master

**qcodes commit**
cfc97ec2090ed3620634affa3177419dddb6abae

Start by reading qcodes/instrument/base.py and qcodes/instrument/function.py, then inspect the existing behavior noted in qcodes/tests/test_instrument.py. Determine how collisions between parameters and functions should be handled, and add coverage showing that neither is silently overwritten once the intended behavior is decided.

Written by the indexing model from the issue text.