Ncrystal

* Fix bug in LoadedMaterial.xsect/.macroscopic_xsect methods.

* Add several new convenience methods to MiniMC results and histograms.
* Fix bug in MiniMC histogram error propagation.
* NCMATComposer gets new expert-only methods for adding CUSTOM_ sections
or raw text to NCMAT data.
* NCMATComposer.from_hfg sets plotlabel from title.

* Make ncrystal_hfg2ncmat functionality available in Python API as well in
the new NCrystal.hfg2ncmat module. Also add NCMATComposer.from_hfg(..)
factory function to access the functionality.
* Fix bug in Python-C++ message handler infrastructure which potentially
caused message handler to be set more than once.

* Fix PhononDOSAnalyser (github issue 187).

* Fix NCrystal.cifutils plot labels with underscores in filenames.
* Fix MiniMC plotting.

* Prevent rare error in the MiniMC framework, which happened when acos was
evaluated on a value which due to numerical instabilities was ever so
slightly larger than one.
* Minor tweaks to MMC parsing and plotting code.