Blocks of type "evaluate backward" are not correctly simulated

The testsuite does not currently have a block of this type (which typically appears when there is a purely forward variable).

Tasks:

• Fix the bug in MATLAB mode
• Fix the bug in bytecode
• Add a test in the testsuite

The test can simply consist of adding an equation to block_bytecode/ls2003.mod, which would read as: pure_forward = 0.9*pure_forward(+1) + e_pure_forward;. The e_pure_forward variable needs to be shocked, preferably in the end of the simulation sample.