Add initialization method with real data for LCT model

[lenaploetzke]

Feature description

Add method to get initial numbers from real data for LCT model

Additional context

Copy code from #680
Files:

• cpp/memilio/io/epi_data.h
• cpp/tests/test_epi_data_io.cpp
• cpp/models/lct_secir/parameters_io.cpp /.h
  and related files

Checklist

• Attached labels, especially loc:: or model:: labels.
• Linked to project.