Add schema validation for json data

[CunliangGeng]

Fix #165.

Major changes:

• added json schemas and related utils to new folder src/nplinker/schemas
• added unit tests on json schema itself and relevant valid/invalid data to new folder tests/schemas
• added json data validation after loading json data or before saving data to file, i.e. following the load-validate and validate-save patterns
• update some code and test data to match the requirements of json schema

[gcroci2]

All good! jsonschema has very nice functionalities.

The only comment I have is that three schema tests (for GENOME_BGC_MAPPINGS_SCHEMA, GENOME_STATUS_SCHEMA, and STRAIN_MAPPINGS_SCHEMA) have some redundancies (e.g., 'version' is a required property, data_invalid_version, ). We could think about a way to iter over the three schemas and reduce the number of files (and lines of code), but I am not sure if this is worth it, and it's not going to be easy. On one hand indeed, it's useful for a developer to have the three distinct files for testing the three schemas, it makes things very clear.

[CunliangGeng]

We should keep the tests on different schemas separate, in this way the tests are modular and clear, most importantly, it's easier to extend when schema changes.