update README in src

- DB_PORT: an integer representing the port to connect to the database on (default 5432)

- DB_DB: a string representing the database to connect to (default postgres)

- MAX_RETRIES: an integer representing the maximum number of times to retry the connection (default 60)

Usage: `test-db-conn -d <DB_DB> -h <DB_HOST> -p <DB_PORT> -r <MAX_RETRIES>`

- Usage: `test-db-conn -d <DB_DB> -h <DB_HOST> -p <DB_PORT> -r <MAX_RETRIES>`

## Classes

DATABASE_PORT, DATABASE_SCHEMA

These environmental variables are read in using `env.py` function check_environment and have default values assigned.

Best practice is to use `with` to enter the database, i.e. `with Database as db:`

This will "silently" open the database from the `__enter__` function.

## Mixins

Mixins for the Database class found in `database.py` can be found in `./common/mixins/`.

A postgres mixin that uses psycopg2 can be found in `./common/mixins/postgres.py`.

Usage with the Database class would look like: `class PostgresDatabase(Database, PostgresMixin)`.

Similary, a pandas mixin can be found in `./common/mixins/pandas.py`.

Usage with the Database class would look like: `class PostgresDatabase(Database, PandasMixin)`.

## Mixins

## Functions

Within `database.py`:

`is_open` Determines if the database is open or not, returns True if open and False if closed.

`override_connection` Overrides the default connection, useful for using other connections than `psycopg2` for downstream development

`modify_connection_info` Modify a `variable` and set it to `value` in the connection_info

attribute.

Within `postgres.py`:

`open` Explicitly open the database connection and tests a cursor

`query` Passes a user defined SQL query through the psycopg2 interface, extensive error catching.

Within `pandas.py`:

`query_pandas` Return a query as a pandas table. Provided a query, uses the default cursor assumming a PostgreSQL connection.

`make_dataframe` Create a pandas dataframe from a query.

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