Disable caching for discretizations by default

[sdrave]

Currently, the default cache region for discretizations in pymor.discretizations.basic is 'disk'. I propose to change this to None. IMHO, pyMOR should behave as reproducible as possible by default. Having persistent cache entries on disk will change timings in subsequent runs. Moreover, first time users will probably be surprised that pyMOR dumps up to 1GB of data to disk. Re-enabling caching will be as simple as writing

discretization.cache_region = 'disk'

Any objections?

[renefritze]

Sounds good. I'm a little concerned if anybody new will do/find that though.

[sdrave]

At least, we could add an option to the demos ..

[renefritze]

I like that.

[ftalbrecht]

So, in view of #179, one would write

discretization.enable_caching(region='disc')

to enable caching and

discretization.disable_caching()

or

discretization.enable_caching(region=None)

to disable caching again?

[sdrave]

Yes, but I would write:

discretization.enable_caching('disk')

[sdrave]

Done in 4c67dd7.