Move cache folder to separate repos

[pkrog]

Use separate repos to store cache files used for testing and vignettes.
Do not define it as submodule, since it would be installed automatically in Travis, Bioconductor or with remotes::install_github, but define a target in the Makefile for installing it:

install.cache:
   git clone https://github.com/pkrog/biodb-cache cache

[pkrog]

The current cache is already as large as 700MB.
So maybe it would be better to have one repos for each database cache.
The first step would then be to upgrade cache system so each database has its own subfolder (see #329 ).

[pkrog]

Abandoned, cached files are not stored anymore for testing.