Restructure primitives folder and various util functions

[ozzieD]

The current structure of the primitive's folder makes it confusing to work with and harder to scale. @dvreed77 and I think a better structure for the primitives would be:

primitives
│  __init__.py ( remove all api.py since its not needed)
└───aggregation
|   |    __init__.py 
│  └───standard
│     │    __init__.py 
│     │   *each agg primitive listed in its own file*
└───transform
│   │    __init__.py 
│   └───standard
|       |    __init__.py 
│       │   *each trans primitive listed in its own file from `transform_primitive.py`*
│   └───cumulative
|       |    __init__.py 
│       │   *each cum trans primitive listed in its own file from `cum_transform_feature.py`*
│   └───datetime
|       |    __init__.py 
|       |    utils.py 
│       │   *each datetime transform primitive listed in its own file from `datetime_transform_primitive.py`*
│   └───latlong
|       |    __init__.py 
|       |    utils.py 
│       │   *each latlong transform primitive listed in its own file from `latlong_transform_primitive.py`*
│   └───rolling
|       |    __init__.py 
|       |    utils.py 
│       │   *each rolling transform primitive listed in its own file from `rolling_transform_feature.py`*

[gsheni]

• Shouldn't we be consistent with having api.py OR not, throughout the codebase?
• Won't we have a ton of files with this change? If a dev has to change a primitive or wants to scroll through them, they have to look at multiple files...

[ozzieD]

• Yeah I purposely want to rip api.py out of the codebase if there is no particular reason for it. We would be able to delete many flake8 ignore comments doing this.
• Yeah we would have more files but finding the primitive I want to modify would be much easier imo. Instead of having to scroll through huge files we now would have a collapsed view of the primitives via their file names. This would also allow us to namespace our utility functions better.