Remove slopeTypeIndex attribute and associated code

[wknoben]

Feature Requests

• Description of feature
  The attribute file requires slopeTypeIndex to be specified as a result of using the Noah-mp code for lookup tables. This variable does not appear to be used anywhere in the code (anymore?). Removing the need to specify this variable will clean up the code.

• Description of the problem the feature addresses
  Unclear code, need to specify a variable that serves no purpose.

• Will the proposed feature be backward compatible?
  Tentatively yes, assuming that it is OK if an attributes.nc file contains more variables than the code looks for. If so, existing attributes.nc files that contain a slopeTypeIndex value should still function after this variable has been removed from the code.

• Will the proposed feature change the science results of SUMMA?
  No.