Arithmtic on params

[mahyarsamani]

I did a little check on how params are defined, I'm not sure if it is possible to do arithmetic operations on params, it would be nice if I could do this:

self._mem_size = '2GB'
self._cache_size = self._mem_size / 256

I think there are some use cases to this, for example in my gups test I need to break down some part of memory to smaller chunks, it will be nice if I could do it without converting _mem_size to int and converting _cache_size back to str.

[mahyarsamani]

Also it would be nice to be able to do Addr = Addr + MemorySize or MemorySize = Addr - Addr.