Replace pianoroll with `FrameSequence`, rows with `Frame`

[vabold]

As a rough sketch of what our new infrastructure would look like:

class Frame:
    def __init__(self, raw: List):
        # Initialize frame information based on the CSV line

class FrameSequence:
    def __init__(self, filename: str):
        self.frames = []          # List of initialized Frame classes
        self.filename = filename  # Name of the CSV file

        self.refresh()

    def refresh(self) -> None:
        # Load the CSV and iterate self.process() over each line

    def process(self, raw_frame: List) -> None:
        # Process raw frame (list from reading a line in the CSV) and add it to the sequence

[ximk]

Resolved in #10.

[vabold]

Resolved in 2994858c3ddf5e4346b0a4b6963daaf27c069ef5.