Removed compilation warnings.

[rturrado]

• backend.cc: removed unused variable 'wait'.
• functions.cc/.h: changed 'expOpCnt' parameter type from Int to UInt, to avoid a signed/unsigned comparison. The same for the for loop variable 'i'.
• new_to_old.cc: removed unused variable 'for_loop'.
• old_to_new.cc: initialize 'assignable' boolean variable, to avoid its use when uninitialized.
• prim.h:
  • Warnings complained about 'data' being initialized from an uninitialized 'ncols'. This may be a false positive, due to the constructor parameter being called like the member variable. Anyway, it can be easily fixed by renaming the parameter (having parameter names equal to member names can be quite confusing).
  • made constructors receiving one single parameter explicit.
  • removed an extra semicolon at the end of the initialize function template.

[rturrado]

All these changes have finally gone into the main branch via other PRs.