TAWS generic alert parameter and test

Introduces a derived parameter TAWS Alert formed from concatenation of
whatever TAWS parameters are available, then leading to a single KPV to
monitor the duration. We can add reaction strength etc. for this single
parameter. Also, as TAWS alerts that are wrongly named will produce a
None slice list, this is trapped within the slices_or library routine
with a suitable reminder.

Minor change in merge_two_parameters log output to illustrate the %
valid values. My Python isn't good enough to output a "%" character
without getting muddled with delimeters, so someone else can tidy that
up please!