lcmgen: Emit file level comments

[judfs]

Parse any comment before the package statement in the .lcm file and apply it to the top of generated source files right after the 'generated with LCM' statement.

Does not apply if the package is defined via CLI flags.

Implemented for:

• c
• cpp
• java
• python
• go
• lua
• c#

See test/types/lcmtest/comments_t.lcm and
build/test/types/lcmtest/comments_t.*

[judfs]

I just realized multiple types can be defined in 1 .lcm file. I did not account for that

[judfs]

https://lcm-proj.github.io/lcm/content/lcm-type-ref.html#type-specifications suggests (and also demands) one file per type. So should multiple types per file even be supported?

[judfs]

@ihilt any thoughts? I had put off implementing the other languages until I got feedback.

[ihilt]

@ihilt any thoughts? I had put off implementing the other languages until I got feedback.

I'd stick with one type per file but I'm not tracking this super closely at the moment. @nosracd do you have any objections to that?

[nosracd]

I'd stick with one type per file but I'm not tracking this super closely at the moment. @nosracd do you have any objections to that?

So to make sure I'm tracking:

The LCM documentation implies that there should be one struct per .lcm file, although it does not explicitly state it as a rule. The most strong wording I can find is this bit (talking about the struct temperature_t):

This declaration must appear in a file named temperature_t.lcm.

However, lcm-gen does support multiple structs per file and there's a chance that existing systems use it that way.

But, this PR will not affect any of the existing support for multiple structs per file. Instead, it's adding a new feature: file-level comments. And the current state of the PR is that it only supports file-level comments for .lcm files that only contain one struct per .lcm file. For example, if there are two structs defined in a .lcm file (foo_t first then bar_t) then only the output files for foo_t will have file-level comments.

Assuming I'm tracking all that correctly, then I think it's fine to only support file-level comments for .lcm files that have one struct per .lcm file. But I think the documentation should be more strongly worded to recommend that users only have one struct per .lcm file.

[nosracd]

Looks good to me. Noticed one minor typo but just let me know whenever you want me to hit the merge button

[judfs]

Looks like accepting the suggestion didn't cause formatting errors so should be good to go