Facility for fixed, user defined tree

[cerebis]

Rather than only procedurally generated trees, we should reintroduce the ability to have defined trees. Either as a file reference or string in YAML config. Concern that the YAML string may prove problematic when creating filesystem instance -- due to illegal character use. For file references, we must either impose absolute paths or add logic for relative ones when in working directories.

Trees defined as such then have ntaxa defined also. Therefore this detail should move into tree parameter of Clade. A tree is then either a set of grouped parameters

For future utility with multiple algorithms, this would be more complete

tree:  [ ntaxa:5, algo: [ name: 'birth-death', [birth: 0.9, death: 0.5] ] 

OR explict definitions either as file or string

tree: [file: 'a/b/c.nwk']

OR

tree: [string: '(A:0.1,B:0.1);']

[cerebis]

This is implemented on the branch: experiment

[cerebis]

05a8e4f7395be85769ae3c577b8aeb47012f2e01