Support for mixed-case calendar strings

Iris currently only supports all-lowercase calendar strings, i.e. `'gregorian'` is supported while `'Gregorian'` would not be. This strictly adheres to the calendar strings quoted in the CF spec in [Section 4.4.1](http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.7-draft1/cf-conventions.html#calendar "Section 4.4.1. Calendar"), where the calendars are all-lowercase strings. However [Section 2.6](http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.7-draft1/cf-conventions.html#idp4850176 "Section 2.6. Attributes") of the CF spec states:

  > When this standard defines string attributes that may take various prescribed values, the possible values are generally given in lower case. *However, applications programs should not be sensitive to case in these attributes.* (Emphasis added)

Further, the cfchecker tool does not complain about mixed-case calendar strings.

This indicates that Iris is currently being too strict about calendar strings, which is fixed by this PR.

Note that this clarification above may need to be further applied to attributes that are not calendar strings, however determining where this applies is beyond the scope of this particular PR.