Update Colvars library to version 2019-04-26

The following is list of relevant issues fixed and improvements:

Fix forces and missing output of runtime histogram for histogramRestraint
https://github.com/Colvars/colvars/pull/246

Use fix_modify to add configuration to Colvars:
https://github.com/Colvars/colvars/pull/216

Fix componentCoeff and name not working with orientationAngle components:
https://github.com/Colvars/colvars/issues/213

Fix 1-timestep offset with extendedLagrangian:
https://github.com/Colvars/colvars/pull/210

Changes to improve compiler support:
https://github.com/Colvars/colvars/pull/203

Fix ignored anisotropic cutoff3 for groupCoordNum:
https://github.com/Colvars/colvars/pull/202

New dipoleMagnitude variable:
https://github.com/Colvars/colvars/pull/198

Parser improvements:
https://github.com/Colvars/colvars/pull/196

lib/colvars/colvarbias.h

@@ -96,6 +96,9 @@ public:
   /// \brief Parse config string and (re)initialize
   virtual int init(std::string const &conf);
 
+  /// \brief Initialize dependency tree
+  virtual int init_dependencies();
+
   /// \brief Set to zero all mutable data
   virtual int reset();
 
@@ -181,7 +184,7 @@ public:
   static std::vector<feature *> cvb_features;
 
   /// \brief Implementation of the feature list accessor for colvarbias
-  virtual const std::vector<feature *> &features()
+  virtual const std::vector<feature *> &features() const
   {
     return cvb_features;
   }
@@ -220,7 +223,7 @@ protected:
   bool                     has_data;
 
   /// \brief Step number read from the last state file
-  size_t                   state_file_step;
+  cvm::step_number         state_file_step;
 
 };