ENH: Simplify classes, move out non-class private methods, remove strict-handling, make functions private and rename accordingly

FIX: set destination projection to destination dataset (fixes #368) (#369)

* FIX: set destination projection to destination dataset (fixes #368)
* FIX: set dst_proj if dst_proj is not None

ENH: improve OdimH5- and CfRadial-readers, apply keywords (eg. for chunking, georeferencing), introduce two classes for holding open netcdf-filehandles (also for properly closing), only hold sweep-data in Dataset-dict, workaround Unidata/netcdf4-python#945, properly load multiple OdimH5 files into one volume (DWD one sweep one moment files), several simplifications (#367)

ENH: always write all available dataset-'how' attributes when exporting `to_odim` (#366)

FIX: correct typo GDAL_VERSION env variable

FIX: correctly apply 'a1gate' when saving 'to_odim'

MNT: fix Travis-CI conda handling

MNT: use GDAL_VERSION within TravisCI

FIX: skip NaT values to correctly retrieve start and end times

Hotfix/gdal3 (#357)

MNT: Add gdal v3 build to travis CI, remove strict channel priority (appveyor) (#358)

FIX: build sdist first

FIX: fix tests to work with windows CI (appveyor)

MNT: add `appveyor.yml` for windows CI

FIX: add `silent` as kwargs switch to silence gdal ProgressBar, check for windows to silence anyway

MNT: add files to MANIFEST.in (to be included in sdist)

FIX: use az/el per ray, if available from odim file

ENH: disambiguate GAMIC moment names

ENH: use new function `get_attrs_and_props` to spead up index (and weights) retrieval

ENH: create spatial index and attribute index files on temporary datasets