This reverts merge request !297

added variable_tag to MemAccess; GlobalMemAccessCounter tracking variable tags for tagged global variables

Switching to *None* is not ideal, because *None* is already used in
temp_var_type to represent no value.

Hence, this change introduces an "optional" object wrapper to loopy
and uses it to represent the absence of a temp_var_type in an
assignment instruction.

now counting lid_strides, gid_strides, and variable name in LocalMemAccessCounter; updated tests accordingly

switched lid_strides from list to dict in order to distinguish between two cases: (1) where an lid was found with stride 0 and (2) where no lid was found