ContextDB, refactor and cleanup

[rizac]

Follows #126 with some added features. In a nutshell:

• Implement a better ContextDB class for usage in residuals computation. The class is more friendly to other potential data libraries (e.g. pandas dataframes)
• Fix trellis test failing (modified JSON data accordingly)
• Remove a lot of unused modules related to the pytables library. This includes removing pytables (and PyYAML) from setup.py (only openquake is a dependency now)
• Move check_gmpe_list from trellis_plot to sm_utils, (check_gmpe_list is used also in residuals computation), move rarely used, surface-related code from sm_utils to the new surface_utils module
• PEP8 adjustments in several parts of the code

[g-weatherill]

Looks good to me and nice to be able to remove the Pytables dependency