Implement `get` and `find` methods for units (and maybe other non stand-alone entities)

[msrocka]

See the discussion here: https://ask.openlca.org/2667/error-when-trying-to-get-a-unit-by-id-with-olca-ipc

[hmwakeling]

Just to add my 2 cents, I would find this enhancement very useful in my work.