read functions should throw error for unknown engine

[tlukkezen]

When the value of the engine argument for the read_cpt and read_bore functions is not recognized, the default value is used.

Example to reproduce

Input:

from pygef import read_cpt
read_cpt(file="non/existing/file.gef", engine="non-existing-engine")

Response:

lxml.etree.XMLSyntaxError: Start tag expected, '<' not found, line 1, column 1

Expectation

The expectation is to get an informative error about the enigne value.