add an option to cache selection in memory

[bombrun]

It seems that vaex is not caching selection in memory. I think it will be useful to have an option to cache a selection in memory. It will avoid to go trough all the data again when working on a subset.

One should manage memory limitation, by example by setting a memory buffer limit. One should be warned if the selection exceed buffer limit.

[maartenbreddels]

Actually it should, but only 'named' selections. For instance

ds.plot("x", "y", selection="z > 0")

would not cache anything, but

ds.select("z > 0", name="zpos")
ds.plot("x", "y", selection="zpos")

should cache it, so executing the plot a second time should go faster.
Although caching all selections with some memory limit would be an option, I want to leave it out to keep vaex as simple as possible. I hope this will suffice for the moment.

However, I was testing the performance of this, and found it was not being cached. I fixed this, however, don't expect massive speedups for simple selections. Since having a selection means that the data has to be copied (though this may change in the future), it means a performance penalty for selections.

I'll get back to you on this, in combinations with performance improvements for multiple plots, you may see some speedup, which should also fix #10.

[maartenbreddels]

This is working well now (using arctan2 since it's quite expensive to calculate):

ds = vaex.datasets.helmi_de_zeeuw.fetch()
ds.select("arctan2(y,x) > 0")

%%timeit -r5
ds.count(selection=True)

Gives:

10 loops, best of 5: 18.5 ms per loop

While using an unnamed selection (which will not get cached)

%%timeit -r5
ds.count(selection="arctan2(y,x) > 0")

Gives

1 loop, best of 5: 120 ms per loop

6.5x speedup.