Introduce user-defined placeholders and change API
Introduce user-defined placeholders and change API

User-defined placeholders
-------------------------

- The user can define placeholders with regular expressions which
will be parsed when setting *info_via* to *filename* or *both*.
- The parsed placeholders will be returned as attributes in the
FileInfo object.

Major changes
----------------

- Deletion of the retrieve_time_coverage method, use get_info
instead.
- The whole file info objects are cached now instead of only the time
  coverage.
- *time_coverage* is now only valid for single file datasets and can
  just hold a time period. The options *filename* and *content* are
available with the parameter *info_via*. *content* was renamed to
*handler*.
- *times_cache* was renamed to *info_cache*

atmtools/typhon,atmtools/typhon