Improve errors from `convert` function

The error return by the convert function is not helpful at the moment. I stumbled over this when I organized my participant names in an Enum

class ParticipantNames(Enum):
    MASS_LEFT = "Mass-Left"
    MASS_RIGHT = "Mass-Right"

One has to pass, for example, ParticipantNames.MASS_LEFT.value to the API. But I forgot the .values part, which led to an errors similar to

>>> precice.Interface(1,2,3,4)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "cyprecice/cyprecice.pyx", line 87, in cyprecice.Interface.__cinit__
  File "cyprecice/cyprecice.pyx", line 24, in cyprecice.convert
TypeError: Could not convert.

I think it would be good to tell the user which type was received from the API and which types are supported here. If I would have known that I provided a <enum 'ParticipantNames'> instead of a <class 'str'> this would have helped me a lot debugging.

Start in cyprecice/cyprecice.pyx at the convert function, lines 15-24, and reproduce the failure by passing an Enum member where a string is expected. Update the error behavior so it identifies the received type and supported types, then verify the resulting message against the Interface example in the issue.