Resolve "two issues with hRIXS"

Closes #65

See merge request !235

This inlines the centroiding code into the HRIXS class, and also improves the readability.

See merge request !233

Only use bunch pattern table when necessary

See merge request !234

this should make the integration code for hrixs more readable, and also fixes a bug.

See merge request !231

this removes more code from the inner loop. It is more efficient,
but is it also more readable? Probably not.

Manage exception in load_run_values()

See merge request !232

the curvature correction makes that the edges of our integration
range are not straight. We need to integrate over a larger range
then the resulting spectrum would suggest. Before this code we tried
that, but actually failed.