Compute concept by concept distance matrix

[b]

This is server-side infrastructure to support the following workflow 1) User enters keyword search (e.g. ERAD-degradation pathway) 2) System identifies a set of related concepts
3) System produces N by N distance matrix for those concepts 4) Views are produced for that distance matrix (heatmap, binary network, sortable table)

This issue is for step #3 above. For each concept pair, identify distance based on content in SemmedDb. Need to find (a) the number of pmids associated with each concept (C1, C2) and (b) the number of pmids associated with both concepts. Then apply google distance formula to those three numbers.

[b]

Merge with issue #10.