Validates that all required inputs are set and are correct using a set of input validation rules. The inputs are also validated by default when the calculate_base_building method is run.
The validate_inputs method returns an xpath and status for each input that is either missing or not valid. If status of the input is 2 the input is missing but is not required. If status of the input is 1 the input is not valid and an error_message is also returned.