More robust area computations. Raise custom exceptions instead of generic.

• Use a more robust algorithm for computing intersection polygon area that
  ignores intersection polygons that raise MalformedPolygonError in the
  spherical_geometry package. This may result in sub-optimal alignment
  order but in practice, it should have minimal effect
  on the end result. [#205]

• align_wcs now will raise a custom exception of type NotEnoughCatalogs
  when there are not enough input catalogs to perform alignment. [#203]

• XYXYMatch now will raise a custom exception of type
  MatchSourceConfusionError when multipe reference sources match a single
  input source. [#204]