v0.15.0: Release 0.15.0

Radial profiles API changes:
- sdat.rprof and step.rprof are replaced by step.rprofs
- items of rprofs are (values, rad, meta) named tuples
- extra rprof variables are accessible with the same API as regular
  variables
- rprofs.bounds, rprofs.centers, and rprofs.walls offer access to radial
  coordinates
- rprof.get_rprof and the redges extra variable are dropped

Time series API changes:
- time items of tseries are (values, time, meta) named tuples
- extra time variables are accessible with the same API as regular
  variables
- tseries.tslice replaces sdat.tseries_between, tstart and tend kwargs are
  no longer necessary and have been removed
- tseries.time offers access to time coordinate
- time_series.get_time_series is dropped
- time_series.compstat returns the computed values instead of dumping a
  file

Geometry API changes:
- geom.X_coord are replaced with geom.X_centers and geom.X_walls in
  every direction for a better distinction between these two positions
- geom.X_mesh are dropped as keeping in memory every combination of
  walls and centers position is unecessary and consumes a lot of memory
- the extra layer that was added in the horizontal direction is removed

New features:
- implement repr for Step objects
- _StepsView has a new rprofs_averaged attribute
- the new Step.time property is robust against missing time series
- new primordial layer field "prim"

Changes:
- conf.field.interpolate is now False by default to avoid giving the
  impression that the run is more resolved than it is

Internal improvements:
- computation of EXTRA variables is cached
- eliminate the UNDETERMINED unit value, cache stuff with the
  crop descriptor
- remove the InchoateFiles context manager
- use more modern PEP517 tools to produce releases
- remove the stagpy-git link in user space

Bug fixes:
- a bug in StagYY (fixed) led to incorrect file numbers in the xdmf
  file; StagPy learned to work around that issue
- various documentation improvements
- fix axes boundaries on zoomed time series
- reduce padding between field subfigures
- handle recent T_code addition in StagYY output
- fix reading of Yang grid and fields
- location of fields in field.plot_scalar is correct in most cases
  (vz is still off as it lack the extra surface point)

Compatibility:
- drop Python 3.5 support, use f-strings where relevant
- add Python 3.9 support
- bump dependencies
- StagYY legacy output with magic < 9 is no longer supported