Implement rcl_x_is_valid methods

[firesurfer]

In order to track what was discussed in #78 I open this issue tracker.

@wjwwood suggested that in the future it might become necessary/useful to implement functions in the style rcl_x_is_valid .

An example would be: bool rcl_service_is_valid(rcl_service_t * service) which would check if this service is valid. (Perhaps check if it's not null, check if it was initialized, and so on)

[wjwwood]

Thanks!

[allenh1]

@tfoote Is this the kind of thing I could work on?

[tfoote]

Yeah, We've tagged several issues "help wanted" or also "unclaimed" that you could look into.

There are many more, but we tried to pick out some good places for people to get involved.

[allenh1]

Well I'll start implementing this, then!

[allenh1]

For now, I'm putting these in the rcl.h header -- is this appropriate?

[allenh1]

Actually, after further investigation, I'm now thinking they should be implemented in several different header files (bool rcl_service_is_valid(const rcl_service_t * service) should be in service.h, and similar).

Alternatively, I suppose I could put them in a check.h header, but I'll implement them in the above fashion in the meantime.

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First attempt: c5c6d7d

[wjwwood]

Putting them in the same header with the type you're checking seems appropriate to me.