Better caching for computed values

[samtx]

Implement a consistent caching system for computations that are performed once per day, such as

• Satellite ECEF, ECI vectors from propagate()
• Sun ECI vectors
• Satellite visibility from sun
• Earth orientation parameters
• TLE data
• geocoded location data

Potential cache options are:

• A custom cache wrapper with OrderedDict() that saves the results to disk
• HDF5 table with pytables or hdf5py
• A sqlite table with an ORM like peewee or SQAlchemy

This cache option should be written so that it can be easily substituted with a Memcached or Redis database for web apps.

[samtx]

Used standard library shelve module to create a custom cache object with time-to-live implementation.

Implemented in commit 1b0836272ed27373ecc9e458903e32b38f22f170.