The head of the propagation queue was not begin properly reset.

Adding some debugging functions.

Fixing small bugs.

Small fixes in watching list data structure.

- Implementation of bookmark, unbookmark and rollback procedures.
- Minor changes.

- New implementation of watcher lists.

New implementation of a software floating point implementation (sdbl) for consistency across different platforms and compilers.
Removing useless files and compile time options related to variable activity data type (it can only be sdbl).

Expose all options to command line.
Expose search statistics to users.

- Adding a compile time option to use floats for var activity (now it can be either ‘double’, ‘float’ or ‘unsigned’ (default))
- Adding vector of ‘float’
- Adding an option to configure the ratio of learnt clauses to be kept in clause database at each reduction (0 means no reduction).
- Other small changes.