Fix/session name prepartitioned
This MR includes the following:
- Fixes the determination of the session name from pre-partitioned mesh directories.
- Remove code duplication of the determination of the session name into a separate function.
- Removes the m_filename parameter (since it no longer makes sense when we can load multiple files) from the session reader.
- Improves the error messages in parsing parameters and solver info to show the erroneous XML element.
- Fixes a segfault if a parameter such as "Test = 1 2" is given.
- Detect when boundary conditions with same ID appears multiple times.
This fixes tickets #148 and #122 (closed).