Consider string-like data equal despite different data type

[sadielbartholomew]

To implement the changed behaviour agreed to be appropriate for Data.equals and non-numeric array inputs in the mini-thread NCAS-CMS/cf-python#254 (comment), both for cfdm itself and as passed downstream to cf-python. Namely:

we should allow any two string-like data types to be equal

noting this is (knowingly) in contrast to the behaviour we have chosen for numeric data, where different (numeric) data types will be considered unequal, even if the same data is the same.