Generate MARC bib record - 300 field

Purpose
All the inventory instance physical description data should be mapped to 300 $a MARC field in accordance with the recommended mapping. The data is formatted consistently with parsedRecord.json

For example:

 "physicalDescriptions": [
    "xi, 322 pages, 16 unnumbered pages of plates : illustrations ; 20 cm"
  ]

will create:
300 $a xi, 322 pages, 16 unnumbered pages of plates : illustrations ; 20 cm

Acceptance criteria
1. The generated record is short-lived and generated each time it is requested
2. The record is saved in .mrc file and contains correctly populated data
3. If the fields are not populated in the inventory instance record, the fields are not added to the MARC record
4. If the inventory record has multiple publication entries, each entry will have separate 300 field with both indicators left blank
5. Unit tests are written
6. API tests are written