`CSV` adapter difficult to use and not very flexible

Discovered during the hackathon, the current CSV adapter is not great. It was difficult to map multiple columns as datetimes, it basically presupposes a "symbol column", and it doesnt allow for returning just e.g. a dict of values in the row. Here is a naive alternative built for the hackathon to read Citibike historical CSV data:

import csv as pycsv
from datetime import datetime

from csp import ts
from csp.impl.pulladapter import PullInputAdapter
from csp.impl.wiring import py_pull_adapter_def


class CSVAdapterImpl(PullInputAdapter):
    def __init__(self, filename: str, datetime_columns: list = None):
        if not datetime_columns:
            raise Exception("Must provide at least one datetime column")
        self._filename = filename
        self._datetime_columns = datetime_columns
        self._csv_reader = None
        self._first_row = None
        super().__init__()

    def start(self, starttime, endtime):
        super().start(starttime, endtime)
        self._csv_reader = pycsv.DictReader(open(self._filename, "r"))

        # fast forward to first record
        while True:
            try:
                row = next(self._csv_reader)
                time = datetime.strptime(
                    row[self._datetime_columns[0]], "%Y-%m-%d %H:%M:%S"
                )

                if time < starttime:
                    continue

                for dtc in self._datetime_columns:
                    row[dtc] = datetime.strptime(row[dtc], "%Y-%m-%d %H:%M:%S")
                self._first_row = row
                break

            except StopIteration:
                return

    def stop(self):
        self._csv_reader = None

    def next(self):
        if self._first_row is not None:
            ret = self._first_row[self._datetime_columns[0]], self._first_row
            self._first_row = None
        try:
            row = next(self._csv_reader)
            time = datetime.strptime(
                row[self._datetime_columns[0]], "%Y-%m-%d %H:%M:%S"
            )
            for dtc in self._datetime_columns:
                row[dtc] = datetime.strptime(row[dtc], "%Y-%m-%d %H:%M:%S")
            return time, row
        except StopIteration:
            return None


CSVAdapter = py_pull_adapter_def(
    "CSVAdapter", CSVAdapterImpl, ts[dict], filename=str, datetime_columns=list
)

Start by reading the current CSV adapter alongside csp.impl.pulladapter.PullInputAdapter and py_pull_adapter_def. Use the provided CSVAdapterImpl example as the reference for supporting multiple datetime columns, avoiding a required symbol column, and returning row dictionaries. Done means the adapter API and behavior cover these use cases without losing time-based reading.