Added Properties definition file

Properties for object type and image type added.
Added property creation class methods

Properties are objects containing information about the type and the
value. The value can be a key, a string, a double, float, or an
integer. If the value is a double or float, the property can also
contain errors (both positive or negative) and units (defined by a
key). No unit keys are defined at the moment.
Simple detection of image type from FITS header

Some FITS headers (e.g. from Gemini telescope) contains the OBSTYPE
property with BIAS/SCIENCE/FLAT/DARK as value. But it is not clear yet
if this is a general property used in most FITS files.