relax requirement on final / in include paths

[bob-carpenter]

Summary:

The include path mechanism currently appends the file name to the path, which requires the path to end in /. Instead, check if the path ends in a separator, and if not, add one.

@ahartikainen originally brought this up in: https://github.com/stan-dev/pystan/pull/438#issuecomment-367004027

Current Version:

v2.17.1

[bgoodri]

Also, if it cannot find the #included file, the error message should reference the file name that it did not find.

[ariddell]

Should the milestone for this be bumped? This would be great to get in.

[ahartikainen]

Hi,

could the code also ignore whitespace before and after the statement?

E.g. https://stackoverflow.com/a/216883

[WardBrian]

Issue no longer exists in stanc3