..................
* fixed plotting initial_values
* handling of cache files (problem solved: user control of location for large files from bigger models):
* not strictly limited to *symenergy.cache* any longer
* the cache path can be set using prior to the initialization of `Model` and/or `Evaluator`
::
from symenergy import cache_params
cache_params['path'] = os.path.abspath('./symenergy_cache')
* model method `get_results_dict`
* similar to `print_results`, but returns dictionary `{variable_symbol_name: result_expression}`
* additional differentiation
* `print_results` now makes use of `get_results_dict`