WIP: api.SepaImport.create: declare parameter types

This enables automatic (basic) type validation of most parameters.
(Except for T_BOOLEAN, for which no validation is implemented yet in the
generic API helper functions...)

The validation is limited, but should catch many serious invocation
errors -- such as wrong field mappings.

- probably to be merged into original SepaImport commit
WIP: api.SepaImport.create: additional parameter validation

Explicitly validate the frequency unit and interval parameters beyond
the automatic type validation, to catch even more possible errors.

We only handle the parameters which are direcly processed by SepaImport
itself. (And could result in obscure behaviour, or even an infinite
loop.) The parameters only passed on to subordinate APIs are not
explicitly validated here: any necassary validation should be performed
by these APIs themselves.

- probably to be merged into original SepaImport commit