add metadata and units during processing

[cparcerisas]

For FAIR data standards, pypam should add (at least) the following metadata to its output:

General attributes:

• date created

Attributes per output variable/coordinate:

• units
• standard_name
• long_name
• coverage_content_type

[cparcerisas]

Fixed with commit 94f1ba56ba19f95fb0c2347c7327ff193c61ce09

Some extra information will need to be added in the future