Fix FutureWarnings in the code and tests

[erogluorhan]

Closes #710

Overview

Addresses:

• "FutureWarning: The return type of Dataset.dims will be changed to return a set of dimension names in future, in order to be more consistent with DataArray.dims. To access a mapping from dimension names to lengths, please use Dataset.sizes"
• Unpinning of pyarrow to fix "FutureWarning: Minimal version of pyarrow will soon be increased to 14.0.1. You are using 12.0.1. Please consider upgrading."
• DeprecationWarnings about assertEquals() calls in the test files by converting them to assertEqual()
• DeprecationWarnings about assertAlmostEquals() calls in the test files by converting them to assertAlmostEqual()

PR Checklist

General

• [x] An issue is linked created and linked
• [x] Add appropriate labels
• [x] Filled out Overview and Expected Usage (if applicable) sections

[philipc2]

Some of the changes here overlap with what I started looking at in #708. However, feel free to close that one since this one seems more comprehensive than what I started. I'll give this a look tommorow!

[erogluorhan]

@philipc2 thanks for letting me know, and sorry I missed that one, but you are right the changes here are comprehensively addressing multiple warnings as described in the first cell. Yeah, please give it a review when you have time.