UMLR — Reword isComputable

This is woolly just redirecting to 'can be computed'.

Suggest: A ValueSpecification can be computed when all the algorithms, resources and values required to perform a computation are available. So a ValueSpecification would not be computable if an operation without a body was invoked, an off-line processor is required, or a source value is required. isComputable() is intended to be a static determination of capability, so a computation failure such as divide-by-zero or an I/O
failure would not prevent isComputable() returning true, rather they would cause a query
such as realValue() to return no value.

?? if a computation needs a currently locked resource, is it computable? Probably yes, since if the computation waits patiently the result will appear ??

?? Should be three-valued...

True => a computation now will run
False => a computation now will abort
Null => a computation now might abort