Weird behaviour in UserMaterial.get_constants()

In the file `compas_fea2.backends.abaqus.model.materials` line 276, as mentioned in the `TODO` I have added, I am sure `self.constants` does not belong inside the constant list and should be excluded.

```python
def get_constants(self):
constants = []
for k in self.__dict__:
# TODO: I think we should we add constants in the list below?
if k not in ['__name__', 'name', 'attr_list', 'sub_path', 'p']:
constants.append(self.__dict__[k])
return constants
```

I propose this:

```python
def get_constants(self):
constants = []
for k in self.__dict__:
if k not in ['constants', '__name__', 'name', 'attr_list', 'sub_path', 'p']:
constants.append(self.__dict__[k])
return constants
```