Merge PR #218 (Remove redundant code in hco_extlist_mod.F90)

This merge brings PR #218 (Remove redundant code in hco_extlist_mod.F90,
by @yantosca) into the HEMCO 3.7.1 development stream.

Important updates:
- Remove routine GetExtSpcVal_Dr, which was being called from
  overloaded routines GetExtSpcVal_char, GetExtSpcVal_int, and
  GetExtSpcVal_sp.
- The relevant computations are now done in the overloaded routines,
  thus making a single routine called by all 3 overloaded routines
  redundant.

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>

CHANGELOG.md

@@ -8,7 +8,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased 3.7.1] - TBD
 ### Changed
 - Updated version numbers to 3.7.1
-- Make Hg0 emission factors in `hcox_gfed_include_gfed4.H` multipliers  of the CO emission factors
+- Make Hg0 emission factors in `hcox_gfed_include_gfed4.H` multipliers  of the CO emission factor
+- Removed superfluous routine `GetExtSpcVal_Dr` in `src/Core/hco_extlist_mod.F90`
 
 ## [Unreleased 3.7.0] - TBD
 ### Added