Calib: protect against empty matches

This fix comes from a PICARD recipe that mosaicked different sources,
which led to an undefined value being passed to the JCMT isplanet()
method. Upper-casing an undef value produces an empty string which
then matches true against the list of planets, leading to all manner
of undesireable consequences.

Fixes issue #5.
SCUBA2: obtain makemap arguments from dedicated primitive

The map-making primitive gets all the default arguments from a
dedicated primitive: the same should be done for checking the map
bounds. That way the same pixel scale is used for each calculation.

Fixes issue #4.