Check assignation of BEAM, FIELDSOLVER and DISTRIBUTION in RUN command

The attributes BEAM, FIELDSOLVER and DISTRIBUTION in the RUN command take default values when they are not specifically defined. Currently, in the case that FIELDSOLVER is not defined, a segmentation fault is raised. In the case of BEAM or DISTRIBUTION, an exception is thrown, since OPAL does not find the internal attributes of those commands when taking the default value.

Early exceptions should be thrown in TrackRun to guarantee the assignation of these attributes.