/simulated_load POST request updates

[Bill-Becker]

• Fixes POST requests with doe_reference_name input to pass along the lat/long inputs (was not passing those inputs through)
• Makes load_type optional for both GET and POST requests, with a default load_type=electric, consistent with the simulated_load() function in REopt.jl
• Makes the year input not required for inputs with doe_reference_name, as that is 2017 by default
• Improves clarity of conditional required vs optional inputs, depending on if normalize_and_scale_load_profile_input is true or doe_reference_name is input which are key distinct scenarios for /simulated_load
• Adds tests in test_http_endpoints.py to test expected POST request functionality