Switch to CmdStanPy

[maedoc]

Hi, (nice project! I was wanting to take a stab at this, but you already did a good job.) Author of pycmdstan here, glad you found it useful. It was meant to be a minimal wrapper for CmdStan from Python, but there were still some sharp edges here and there. I gently suggest using CmdStanPy which started with pycmdstan and made it more robust, though the same principle is there: it's just calling CmdStan in the background, but has some niceties like progress bars and argument checking.