Change *formta_imany() to better support boxtree's FMM

See merge request inducer/pyfmmlib!8

* Change centers to be indexed by *INDIRECT:
   This allows using the tree's center order.
* Change rscale to be a scalar:
   This allows parallelizing by tree level.
* Add the option to generate "out_arg_only" arguments:
   This avoids creating unecessary input arrays on the user side.

Older clang doesn't support fopenmp

See merge request !7

Add remark about binary wheels to README, thank Isuru [ci skip]

See merge request inducer/pyfmmlib!6

macOS fixes

Closes #1

See merge request !5

fix compilation with gfortran 8.1+

See merge request inducer/pyfmmlib!4

Compile with -Ofast by default

See merge request inducer/pyfmmlib!3