Revert "macroprocessor: interpret arrays passed on the command line as arrays...

Revert "macroprocessor: interpret arrays passed on the command line as arrays instead of strings. closes #1578"

Commit 204d9cd0 fails with gcc 4.9 (I checked that it works flawlessly
with 6.x). The error message is:

MacroDriver.cc:63:24: error: 'class std::basic_string<char>' has no member named 'front'
         if (it->second.front() == '[' && it->second.back() == ']')
                        ^
MacroDriver.cc:63:53: error: 'class std::basic_string<char>' has no member named 'back'
         if (it->second.front() == '[' && it->second.back() == ']')

Ideally we should bump the version of gcc used in the build system, but I will not do that before the
next bug fix release. I suppose it is easier to replace the front() and back() methods.