Identification of CPP directives in UMDP3_checker

The current C pre-processor pattern assumes that there are no spaces between the start of a line and the `#` symbol:

```py
comment_line = re.compile(r"!.*$")
cpp_command_line = re.compile(r"^#.*$")
word_splitter = re.compile(r"\b\w+\b")
```

However, the [GNU C pre-processor manual](https://gcc.gnu.org/onlinedocs/cpp/The-preprocessing-language.html) explicitly says that spaces before the hash symbol are allowed:

> Preprocessing directives are lines in your program that start with ‘#’. Whitespace is allowed before and after the ‘#’. The ‘#’ is followed by an identifier, the directive name. It specifies the operation to perform.

The pattern should probably allow leading whitespace. This doesn't seem to cause problems for the UM, but it may result in unexpected failures in other code bases.