Release 0.5.0

[mcgarth]

This PR completes the refactor of the PyRate workflow to enable easier analysis of correction data and quicker re-computation with different parameter settings

Added

• New functionality "linear_rate" to calculate linear regression of
  cumulative displacement time series for every pixel as part of the timeseries step.
• Script for plotting timeseries and linear_rate output geotiff products using Matplotlib.
  To use, additional dependencies listed in requirements-plot.txt are required.
• Correction data (except maxvar and vcmt) applied to the ifg data is saved to disk
  and re-used on subsequent repeat runs. Corrections are only re-calculated if config
  parameters change between runs.
• MPI parallelisation of APS spatio-temporal filter correction.
• Unit test coverage for refpixel lat/lon to x/y conversion and aps module.

Fixed

• Re-enable ifglksx and ifglksy to be different values, resulting in different
  resolutions in x and y dimensions in multi-looked interferograms.
• Re-enable orbfitlksx and orbfitlksy to be different values, resulting in different
  resolutions in x and y dimensions during network orbit correction.
• Screen messages from main process only during MPI runs.

Changed

• process step has been renamed correct. Stacking and timeseries have been removed from
  this step and are now invoked by separate timeseries and stack command line options.
• Processing of coherence files by conv2tif and prepifg is now triggered by the presence
  of cohfilelist in the config file. If the list is present, multilooked/cropped coherence
  files are saved to disk, regardless of whether cohmask is 0 or 1.
• Parallelisation capability is refactored - MPI and multiprocessing both now use a common
  tiling framework for stack, timeseries and mst algorithms.
• Introduced a simplified and standardised file naming format for files produced by the
  prepifg step. Info previously saved in the filename (e.g. ifglksx, ifglksy, ifgcropopt)
  is now saved to the geotiff header instead.

Removed

• Redundant tscal config parameter was deprecated - not needed now there is a timeseries
  step invokable on the command line.
• Pillow, cython and glob2 dependencies - not used in main workflow code
• Deprecated function pyrate.prepifg_helper.prepare_ifgs, which is no longer needed.

[mcgarth]

Thanks for your contributions to this release @basaks @chandra2ga @adeane-ga @tfuhrmann !