Reduced memory consumption and convenience functions

Major Features and Improvements

• reduce the memory footprint on (some) fits, especially repetitive (loops) ones.
  Reduces the number of cached compiled functions. The cachesize can be set with
  zfit.run.set_cache_size(int)
  and specifies the number of compiled functions that are kept in memory. The default is 10, but
  this can be tuned. Lower values can reduce memory usage, but potentially increase runtime.

Bug fixes and small changes

• Enable uniform binning for n-dimensional distributions with integer(s).
• Sum of histograms failed for calling the pdf method (can be indirectly), integrated over wrong axis.
• Binned PDFs expected binned spaces for limits, now unbinned limits are also allowed and automatically
  converted to binned limits using the PDFs binning.
• Speedup sampling of binned distributions.
• add to_binned and to_unbinned methods to PDF

Thanks

• Justin Skorupa for finding the bug in the sum of histograms and the missing automatic
  conversion of unbinned spaces to binned spaces.