make directory if missing on write

The TDS write method will create the output directory if the directory
has not been created.
add core Genome module

New core namespace is a functional abstraction over the database layer
which will contain all domain logic.
refactor wobble content histogram into core

The wobble histogram computes the result then stores the value in a
histogram metadata file using the TDS service.
add feature histogram and diff histogram to core

The feature histograms and diff histograms generate results then
store the results in a metadata file. Refactor domain logic from web.
add stats to genome core

Generates gc content and noncoding gc content and saves the statistic
in metadata file. The has_statistic checks if the statistic exists in
the file.
rename gc content function to get_gc_for_genome

The name better describes how the function is being called.
refactor get_gc_for_genome

Use stats routine in core genome and remove old generation method.
refactor noncoding gc stats function

The noncoding gc function uses the core genome stats function to
generate the result and store the result in the metadata file for
future reuse.
load stats if precomputed

Check the metadata file for precomputed statistics then load the
precomputed results otherwise generate the statistic lazily.