bin/openquake:

Added '--export-type' command line option.

Options are 'xml' and 'geojson'.
export/hazard:

Cleaned up export interface to more easily support extension for
multiple export formats.
calcs/hazard/general:

Updated the `export` method to use the new hazard export interface
(which requires the caller to specify an export format).
export/risk:

Cleaned up export interface to more easily support extension for
multiple export formats.

It is also now much more consistent with the hazard export module
and associated interfaces.
calcs/risk/base:

Updated calculator `export` method to use the new risk export
interface.
tests/export/hazard_test:

Factored out export (XML) file validation into the `check_export` util
function.
tests/calcs/risk:

Updated calls to the base risk calc `export` method. The new expected
arguments are now consistent with the hazard calculators. (Instead of
simple a boolean, the type/format of exports must be specified.)
tests/export/risk_test:

Updated/fixed tests to match the new risk export interface.
export/hazard:

Removed unused import.
Revert "bin/openquake:"

This reverts commit 6b52692467eec895564058873c83ed05231458f0.
bin/openquake:

Update `export` calls from the command to specify export type.

For now, this is just hard-coded to 'xml'.
export/hazard:

Made the `export_type` arg of `export` optional, defaulting to 'xml'.
This is so we cause so much disruption to users of this function,
namely QA test code.
export/risk:

Made the `export_type` arg of `export` optional, defaulting to 'xml'.
This is so we cause so much disruption to users of this function,
namely QA test code.
export/risk:

Documentation cleanup.
export/risk:

Whitespace cleanup
calcs/base, hazard/general, risk/base:

Factored out the common export code (the ``export`` method) to the base
Calculator class.
Merge pull request #1226 from larsbutler/geojson-export-prep

GeoJSON export prep

[r=matley,micheles,pslh] [f=*1198843]