Pyvolve

Added argument for both the newick and evolver modules. This argument scales the entire tree (i.e. all branch lengths) with a given multiplier. Note that it is provided in both modules for different general purposes: In newick, it scales tree for all future use. In Evolver, it scales tree only for this particular simulation, so it's useful for replicates each of which have a different scaling factor.

Updated model flag specification in phylogeny, as requested by I. Davydov. Can now provide model flag names with hashtags or underscores.
By providing leading and trailing symbols, e.g. modelflag or _modelflag_, the model flag is propagated to all descendants.
Else, if only a leading underscore, e.g. modelflag or _modelflag, the model is NOT propagated to descendants and hence applies only to that branch.

Addressed implementation issue with gamma-distributed heterogeneity.

Documentation updates with instructions for ancestral sequence evolution.
Additional sanity checks on provided model, partition objects to the partition, evolver modules, respectively, with some added flexibility for providing this information.

Minor bug fix.

Addressed issue where scaling slightly off for heterogeneous codon models in which the dN/dS categories are not equiprobable.