Update based on reviews from core team

[yasahi-hpc]

Comments from core.

• 3 Execution spaces can coexist, SFINAE needs to be improved #79
• Error handling let them know which line causes the error #80
• Avoid variables or functions starting from under score #81
• Function with execution space should be asynchronous. SYCL backend performs a barrier inside #82
• _fft should be refined not to include Execution space Use execspace from Plan. #83
• nvcc version should be 11.0 #84
• support plan: latest two releases #85
• LICENCSE: Apache 2 LLVM exception #86
• Cleanup Plan class. Disallow copies. #90