XMILE operators should be case insensitive

[benslavin]

The XMILE specification states that "identifiers MAY use any mixture of uppercase and lowercase letters, but identifiers that differ only by case will be considered the same" (3.2.2.2) and that "the operator names AND, OR, and NOT [...] are reserved identifiers." (3.2.2.5)

Given this, the operators should be treated in a case insensitive manner. However, pysd currently only recognizes XMILE operators (AND, OR) when presented in lowercase.

When using uppercase, for example...

Given the input equation:
<eqn>( IF true_input AND false_input THEN 1 ELSE 0 )</eqn>

A parsing error will be generated of the form:
parsimonious.exceptions.VisitationError: KeyError: '012e2740d4ee4180969f747b5b71c02bAND756dca4992c74317b8c54296494a7758'

Note the 'AND' in the middle of the generated key, which has been improperly handled.

I'll generate a PR to address this shortly.