Add test cases for imported functions

Add test cases for imported functions. Still to add code to pass these
tests.
Remove trailing newlines from test cases

Remove trailing newlines from new test cases in
`FSIC/parser/tests/test_code.py`.
New `substitute()` function

New `substitute()` function in `parser.code` to carry out
regular-expression substitutions. This implementation ensures that
Python keywords and function calls are left intact, rather than treated
as model variables.