Parser throws System.IndexOutOfRangeException when parsing {annotation}/{annotation}

[alexzautke]

When parsing a metric consisting off {annotation}/{annotation} (1/1) an System.IndexOutOfRangeException is thrown while parsing the second annotation in the term. The current code assumes that the numerator is a proper unit when the denominator is an annotation.

Test: {reads}/{base}

[alexzautke]

Added a ticket to the UCUM tracker system as well, as there is no official test for this combination of annotations.
https://ucum.org/trac/ticket/5776