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.

See merge request !426