ALL: allow to change number of threads in kdtree

[Marcello-Sega]

Please make sure that your pull request satisfies the following requirements:

• in environments with limited resources (e.g. a job in a cluster queue) the default option of kdtree to use all threads can be nefarious:

ERROR; return code from pthread_create() is 11
       Error detail: Resource temporarily unavailable

One can now pass the option workers as in

inter = pytim.GITIM(u,workers=1)

to limit the number of threads. Note that this option has no effect on the multiproc one. This level control is probably too fine and should be revised.

[Marcello-Sega]

bump