Updated conan to use version 3.1.0.

This means all processes must now return ExecutionResult objects rather than success flags.

Also updated Assembler interface so that it includes a getBestAssembly method.  This is instended to return the most contiguous assembly if the assembler creates several assemblies.  For example, it returns scaffolds, rather than contigs, and contigs rather than unitigs.  All implementing classes have been updated accordingly.