Improve multicheck interface

[ZaxR]

Currently multi_check requires unnecessary specification when there are no args and doesn't have an easy way to add small custom functions.

For example, an empty dict is needed as a value for has_no_nans:

...
checks_dict = {ck.has_no_nans: {}, ... }
ck.multi_check(df, checks_dict)

Also, you need to make a function for a user-defined check so that it can be fed in as a key to the checks_dict. Often that's just extra boilerplate, and usually will need an empty dict value like has_no_nans.