Add support for 'like' comparator in the query() function of PdsClient class

[stevenlujpl]

In order to query text fields using PDSC, we want to add support for like comparator in the query() function of PdsClient class.

pdsc/pdsc/client.py

Lines 141 to 173 in 74893bb

	def query(self, instrument, conditions=None):
	"""
	Supports a generic query of observations based on metadata
	:param instrument:
	PDSC instrument name
	:param conditions:
	a collection of tuples indicating query constraints; each tuple
	should contain three entries:
	- metadata variable name
	- comparator (``'='``, ``'<'``, ``'>'``, ``'<='``, ``'>='``)
	- value
	A SQL-like query will be performed with a logical AND of the
	specified conditions
	:return: a list of :py:class:`~pdsc.metata.PdsMetadata` objects
	corresponding to observations matching the specified query
	conditions
	>>> import pdsc
	>>> client = pdsc.PdsClient()
	>>> metadata = client.query('hirise_rdr', [
	... ('corner1_latitude', '>', -0.5),
	... ('corner1_latitude', '<', 0.5)
	... ])
	.. Warning::
	This function currently assumes non-adversarial inputs; the current
	implementation allows a potential SQL injection attack.
	"""