`.sim()` should graph using matplotlib and subplots

[eric-downes]

Necessary:

Use matplotlib for automated graphing of any df columns

optional argument: graph:tuple[str,...]=None

• with no parameters passed: graph df.t vs df[var] for every col in df not t
• graph every column in graph & df.columns

optional argument strict:bool=False:

• if not graph <= df.columns raise ValueError