[SPARK-3389] Add Converter for ease of Parquet reading in PySpark

https://issues.apache.org/jira/browse/SPARK-3389

Author: Uri Laserson <laserson@cloudera.com>

Closes #2256 from laserson/SPARK-3389 and squashes the following commits:

0ed363e [Uri Laserson] PEP8'd the python file
0b4b380 [Uri Laserson] Moved converter to examples and added python example
eecf4dc [Uri Laserson] [SPARK-3389] Add Converter for ease of Parquet reading in PySpark
SPARK-CORE [SPARK-3651] Group common CoarseGrainedSchedulerBackend variables together

from [SPARK-3651]
In CoarseGrainedSchedulerBackend, we have:

    private val executorActor = new HashMap[String, ActorRef]
    private val executorAddress = new HashMap[String, Address]
    private val executorHost = new HashMap[String, String]
    private val freeCores = new HashMap[String, Int]
    private val totalCores = new HashMap[String, Int]

We only ever put / remove stuff from these maps together. It would simplify the code if we consolidate these all into one map as we have done in JobProgressListener in https://issues.apache.org/jira/browse/SPARK-2299.

Author: Dale <tigerquoll@outlook.com>

Closes #2533 from tigerquoll/SPARK-3651 and squashes the following commits:

d1be0a9 [Dale] [SPARK-3651]  implemented suggested changes. Changed a reference from executorInfo to executorData to be consistent with other usages
6890663 [Dale] [SPARK-3651]  implemented suggested changes
7d671cf [Dale] [SPARK-3651]  Grouped variables under a ExecutorDataObject, and reference them via a map entry as they are all retrieved under the same key
Minor fix for the previous commit.