Add table_of_functions template; start using it

In order to avoid having to maintain the tables of functions every time
we add a new function, I decided to do it programmatically using the new
table_of_functions template.

In this commit I demonstrate how to use it in
h2spatial-ext/affine-transformations. The following commits will move
the descriptions to the corresponding files.
Update aggregates
Update h2spatial-ext/geometry-conversion
Update geometry-creation
Update distance-functions
Update h2spatial-ext/predicates
Update h2spatial-ext/properties
Update h2spatial/geometry-conversion
Update h2spatial/operators
Update h2spatial/predicates
Update h2spatial/projections
Update h2spatial/properties
Add empty description variables
Merge pull request #103 from agouge/template-function-tables

Add table_of_functions template