Remove Pipeline Executor

[runette]

Changes to Move from pdal::PipelineExector to pdal::PipelineManager since the former is deprecated.

Also - extended the ABI to include streamable execution.

[runette]

@hobu or @jimmysoda - since this is a relatively major change - it would be very good to get a review if you have the time

[hobu]

This is usually done with compile-time definitions instead of sticking it here, but i'm not much of a windows dev

[runette]

In the absence of any comment from @jimmysoda - I am going to go ahead and merge

[runette]

@abellgithub - could you possibly "spot me" as they would say in climbing (i.e. check my reasoning) since I am to a large extent reverse engineering some stuff here :

About null terminating strings etc as per review comment5s:

1. looking at the C# bindings it is clear that they, at least, DO need guaranteed null terminated strings,
2. c_str() will guarantee a null terminated string except, of course, that strncpy does not then preserve that if size of string > size of buffer,
3. I am guessing that this is what was happening with the "interesting" initialisation of the buffer - to guarantee the null termination.
4. However - this was inconsistently applied and anyway is not very "readable".
5. So I have changed all instances to :
   a - get the string
   b - resize the string to max length (size -1) (to allow for the null character),
   c - convert using c_str() which should guarantee a valid null terminated string of max length of size.

Is my logic valid?

Thanks