Add support for std::vector containers to global variable system

Supported types are vector<int>, vector<unsigned int>, vector<float>
and vector<double>. A vector variable can be defined by calling and
one of the overloaded instances of THaVarList::Define or the generic
THaVar constructor. Defining std::vector variables via RTTI is not yet
implemented.

Closes #22.

As part of this, THaVar gets a new member variable fDim to hold the
current size of object arrays and vectors.  fDim replaces the
problematic local static variable in GetObjArrayLenPtr() with
per-variable storage. The GetObjArrayLenPtr() function has been
replaced by GetObjArrayLen().  This fixes #26.

Also, change THaArrayString::fNdim from Byte_t to Int_t. Really no
need to be that petty.