Default variability for FmuVariables and allowed combos causality/variability

[kp-dc]

Hello,

Thanks for the great work. I'm playing around with fmu_tools and I noticed the following that might hamper portability of generated FMU's:

1. the default variability for variables whose type is other that real is still continuous.
   https://github.com/projectchrono/fmu_tools/blob/54f901e32abbd2e51b54387bb372316f59885ff9/fmi2/FmuToolsVariable.h#L131
   The specification 2.0.5 (see p. 49) explicitly mentions that it is not allowed (missing at 2.0.4) for variability:

The default is “continuous”.
[Note that therefore, for all variables that are not of type = "Real", a variability
value other than "continuous" has to be supplied with an explicit variability
attribute.]

2. regarding the allowed combos for variable variabilities/causalities, we see here:
   https://github.com/projectchrono/fmu_tools/blob/54f901e32abbd2e51b54387bb372316f59885ff9/fmi2/FmuToolsVariable.h#L212

that the rule only applies for input and not output variables as the comment above mentions.

Similar observation apply for FMI 3.

PS. There is also a missing header at fmu_tools/fmi3/FmuToolsVariable.h: line 29 (gcc 13.3.0, complains for uint64_t):
include