Added tests(GetSize, GetAllocatedSize, GetString) and modified tests (SetString, replace) in StringTest.

[skip ci]

The method was either returning true or throwing an exception, so it can
simply return void or throw an exception.

- There is an RAII wrapper called ScopedInitializer to handle
  finalization due to C++ exceptions and to avoid calls to Finalize in
  all exit branches.
- Fixed signature of MpiCommunicator::Init to match that of MPI_Init.

This is closer to value_type which is used in STL containers.

They are virtual, currently not needed and only complicate things.

Tests for Vector<MultiPrecision> were removed by TO...