Add dimension for Volume

[TRoboto]

What problem are you trying to solve?

Currently volume parsing isn't implemented. This implements the volume dimension.

Check the documentation here for more information on creating a custom dimension.

Examples (if relevant)

This should be used as follows:

from maha.parsers.functions import parse_dimension
sample_text = "الف وعشر لترات او مئة جالون او 44.5 متر مكعب"
result = parse_dimension(sample_text, volume=True)
assert len(result) == 3

Definition of Done

• It must adhere to the coding style used in the defined dimensions, preferably duration/time dimension.
• The implementation should cover most use cases.
• The value of each result should be an instance of ValueUnit.