Improve parset read

Currently, the parset is read twice, resulting in output being logged twice. Additionally, when a full path is not used for the parset file (e.g., with a command like $ rapthor rapthor.parset), the second read of the parset cannot find the parset file, due to a change of the directory done during the first read. This MR fixes these issues and also adds an explicit check for required parset parameters.