add platform select to dense_blas

[marty1885]

Add the ability to select which OpenCL platform user wants to run benchmark on form the command line. So user can benchmark their device when they have multiple platform.

[karlrupp]

Thanks! Would you mind replacing stoi with atoi so that things also work with C++03?

[marty1885]

@karlrupp Sure. I use strtol instead of atoi. atoi is dangerous that it does not check if the user input is valid. I also made it prints a usage message then exit if invalid parameter is provided(not number/ too many).

[karlrupp]

Thank you!