Rename a few methods and a formal argument name

Methods:
empty -> isEmpty
front -> head
back -> tail

Formal Argument:
position -> pos
Add array count method and fix clear to reuse the domain low bound

// returns the number of times 'val' occurs in the array
proc count(val: this._value.eltType): int;

Added a new test that uses count.

clear() now sets the domain to {low..low-1} instead of {1..0}.
Add comments describing the new array vector style methods

Add comments ahead of each of the new array vector style methods. Also, added
_array.idxType to match _array.eltType and switched from this._value.idxType
to this.idxType and from this._value.eltType to this.eltType.