Design odML templating engine

Ability to create templates will be very practical feature. odML templating engine will help users to define templates. c.f. [Python templating ](https://wiki.python.org/moin/Templating#Templating_Engines) and re-use them as needed. A light weight example [here](http://markup.sourceforge.net/).

A use case for example, a type of experiment having performed by different parameters, lab or experimenter. A metadata definition is the same in this case but the field values are different.

See also discussions with @lzehl, c.f #93 and #95.