Improve documentation

[gw0]

Could you improve the documentation, docstring comments and naming of files?

For a newcomer to HDL it took me some time to even figure out what is where and what purpose does it have. Suggestions for name changes:

• sdram.py --> SdramHwSim.py, because it is a Simulated SDRAM model
• sd_intf.py --> SdramHwIntf.py, because it is a Interface on SDRAM hardware side
• host_intf.py --> SdramHostIntf.py, because it is a Interface on host side
• class MySdramCntl --> class SdramCntl, because it is a general, not mine, SDRAM controller with SdramHwIntf and SdramHostIntf interfaces
• test_controller.py --> test_SdramCntl.py
• test_sdram.py --> test_SdramHwSim.py
• modal --> model (common typo in README.md)