added functionality for replace_subject/test/trial when handling mixed spaces

[JHopeCollins]

The replace_* functions in labels.py are missing some functionalities that are needed when handling mixed spaces

1. replace_subject should correctly handle the new object being a ufl.Indexed instead of a Function/tuple of Functions etc
2. replace_test_function and replace_trial_function should be able to take an idx argument to replace single components of mixed spaces.

[JHopeCollins]

Some extra points:

• I added Exceptions when you should have passed an idx to replace_subject but didn't.
• When the parameter combination to the test_replace_subject unit test are incompatible it was previously returning True. pytest was complaining and saying that this would be deprecated soon, so I changed to return None. This could also be changed to calling pytest.skip(<some useful message>) from inside the test.
  I need to think about the parameter combinations because adding the function_or_indexed parameter has doubled the number of tests. Now I think I have a fix I can go back and see if all of these are necessary.
• Should the replace_subject test be checking that the subject has been replaced correctly? I think that currently it just checks that it hasn't thrown.

[jshipton]

This looks great - thanks @JHopeCollins! I have made an issue to remind us to improve the test for replace_subject: #323