Disable DART_ENABLE_SIMD by default

[jslee02]

Motivation

When DART_ENABLE_SIMD is on, DART enables SIMD instructions by adding compiler option -march=native. This compiler option enables all the available SIMD instruction subsets of the local machine. This way has two caveats:

(1) We don't convey the compiler option to the DART dependent projects (e.g., Aikido). So the DART dependent project don't know whether DART is built with SIMD enabled, and it could end up building them with different SIMD options, which may cause segfaults in runtime. I guess #780 is the case.

(2) Even though we pass -march=native to DART and DART dependent projects, still they could be built with different SIMD options if they are built on different machines as @j-rivero mentioned in this comment. This could happen, for example, when Aikido links to DART installed using apt-get (the debian package at ppa:dartsim) and your machine's architecture is not the same with ppa:dartsim's one.

Changes

This PR first pass -march=native as the target compiler option to dart target so that it is automatically added to the consumer project by CMake when DART_ENABLE_SIMD is ON. Secondly, DART_ENABLE_SIMD is now OFF by default. We regard this option is an advanced one and should be used when you know what's happening exactly.

Note

• The reason this wasn't an issue before is that we used only -sse2 option, and this is enabled by default for the modern CPUs. However, -march=native enables more options such as -avx.
• Export each CMake component/target as a separate file #786 should be merged before this is merged.
• I hope this PR resolves Trouble with dart::dynamics::Inertia and unaligned arrays #780.

Please let me know if I missed anything.

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