[parametertreetest] Don't use assert().

assert() works only in debug builds (without -DNDEBUG).  Instead, implement
our own check_assert().
[parametertreetest][fs1527] Check for the error from fs1572.
[parametertreetest][fs1527] Kill int and double overloads of
ParameterTree::get("key", def).

These overloads were using atoi() resp. atof(), effectively ignoring junk at
the end of values.  This led to the situation where a floating point value
"0.5" could be parsed as an int "0" without any diagnostic.