Full definition of the simulation in python

[lsawade]

• Define sources
  • overload read_sources() to read yaml node if it is yaml node and file if it is a file #395
  • create example that defines the source in the specfem_config.yaml
• Define eeceivers
  • overload read_receivers() to read yaml node if it is yaml node and file if it is a file
  • create example that defines the receiver in the specfem_config.yaml