Verify whether manual bounds input is used and correct

The check identifies that the input vector is numeric
and of length 4. An exception stops the function.
A successful check results in setting the boolean
TRUE for the new object 'use_manual_bounds'. This
new object will later nullify the use of the default
choice for the bounds, even if they exist and
'use.calmet.bounds' was set to TRUE, not set to
FALSE, or otherwise not set at all by the user.
Init 'use.manual.bounds' with FALSE

There is no argument to set this logical in the
function (in order to reduce complexity). It is
defaulted in the function body as FALSE and later
changed to TRUE if there is validated input for
use of a manually entered bounding box.
Change order of structures in if statement

Switched the order in the if statement to prioritize
the use of manually set bounds.