Improve perf

[blu-base]

Hi Truong,

I saw your post on cfdonline.com. As the other commenters pointed out, there are some inefficiencies in your code.
Although i am not very experienced in software development either, i applied my current experience with this pull request.

Most changes are regarding changing copying objects to copying their reference only. Moreover, I applied std algorithms where I think they apply. This reduced the call graph by about a magnitude, and reduced the run time by about a half (1.2 seconds to 0.66 seconds on my machine). The serial parts such as writing could be accelerated by avoiding std::endl since this always contains a std::flush. I replaced it with '\n' to keep it in memory as long as possible. Though there is the disadvantage that it leads to higher memory consumption. For very large files, there might be more intermittent flushes necessary.

The slowest part is now the FvmMesh2D::detectNearestNeighbor() method. The current changes to that method are worth about half of the performance gains mentioned above. I believe, this state seems to be not optimal. I didn't attempt to think this part through though...

Additum:
I added a cmake file for my convenience. Since I did not know how you label the executable, I set the placeholder "gmshToX", lacking any good ideas, here =)

Best,
Sebastian

[truong-pham-dang]

Hi Sebastian,

Sorry for late response. Currently your pull request has conflicts. Can you resolve that? I'm very happy to merge your pull request right away.

Best regards,
Truong