Consider dropping input.json as a requirement from Pelicun by moving fields into general_inputs.json

[zsarnoczay]

Problem

• Requiring both general_inputs.json and input.json increases setup friction. The three fields in input.json could live in general_inputs.json.

Proposed solution

• Read these fields from general_inputs.json and expect the user to add this information to it since they need to prepare that file anyway.

Permalink to context in #8: https://github.com/OpenPBEE/Functional-Recovery-Python/pull/8/changes#diff-09ddbe9b47cd801ac2851f14da73cf459f44c2cd8c6d3f2a557541ff2a6bc4b2R578