[Annotations] Required (in)dependent variables of function parameters can be deleted

[pafonta]

NeuroCurator v0.4.4.

Steps:

1. Double-click on a reference with annotations.
2. Click on the 'Parameters' tab in the opened 'Annotations' tab.
3. Click on 'New' to create a parameter.
4. Select 'function' for the field 'Result type' in the panel 'Parameter details'.
5. Click on 'Delete variable'.
6. A dialog window opens to confirm the deletion.

The delete button should be disabled for the first dependent and the first independent variables.

Clicking on 'Yes" and then clicking on 'Save' in the 'Existing parameters' panel gives the following stack trace:

Traceback (most recent call last):
  File "/Users/fonta/conda/envs/nc/lib/python3.7/site-packages/neurocurator/modParamWidgets.py", line 324, in saveParameter
    param = self.paramModStack.currentWidget().saveParameter(relationship, paramId)
  File "/Users/fonta/conda/envs/nc/lib/python3.7/site-packages/neurocurator/paramFunctionWgt.py", line 115, in saveParameter
    return ParameterInstance(paramId, description, [], [], relationship)
  File "/Users/fonta/conda/envs/nc/lib/python3.7/site-packages/nat/parameterInstance.py", line 38, in __init__
    raise TypeError
TypeError
Abort trap: 6