[bugfix] fix build without MPI

Building without MPI has been broken since the merge of !55 (merged).

When MPI is not found, UG_ENABLE_PARALLEL is not defined in CMakeLists.txt and thus, the preprocessor variable ModelP is not defined. This in turn means that the message_buffer and message_buffer_size member variables will not be defined which leads to compilation errors in the accessor methods that use these member variables.

To prevent these compilation errors when not using MPI, I moved the accessor methods into #ifdef ModelP blocks.