Need documentation for Tripal Logger

PR #40

We need more documentation for the Tripal Logger at https://tripaldoc.readthedocs.io/en/latest/upgrade_guide/module_dev.html#tripal-set-message-and-tripal-report-error

list all of the logging levels, i.e. add emergency, alert, critical

How to turn on|off logging

* Logs to:
*   - Drupal Logger (unless specified in options)
*   - Tripal Job log
*   - Server log
*   - Drupal Message (if specified in options)

document placeholder replacement

@param $context
*   The message MAY contain placeholders in the form: {foo} where foo will
*   be replaced by the context data in key "foo". The context array can
*   contain arbitrary data. The only assumption that can be made by
*   implementors is that if an Exception instance is given to produce a
*   stack trace, it MUST be in a key named "exception".

debug only prints if TRIPAL_DEBUG environment variable is set

document options

@param $options
*   An array of options where the following keys are supported:
*     - drupal_set_message: set to TRUE if this message should also be
*       shown as a message for the user to see on the page.
*     - logger: set to FALSE if this message should not be sent to the
*       Drupal logger.
*     - first_progress_bar: this should be used for the first log call for a
*       progress bar.
*     - is_progress_bar: this option should be used for all but the first
*       print of a progress bar to allow it all to be printed on the
*       same line without intervening date prefixes.

tripal / tripal_doc

Need documentation for Tripal Logger #39

PR #40