Add error checking and set defaults for DATM_CPLHIST_DIR and DATM_CPLHIST_CASE

[ekluzek]

Add error checking that DATM_CPLHIST_DIR is set to an existing directory. Also if CPLHIST mode is chosen for DATM (or one of the streams like DATM_PRESAERO) that it's set and not the default of UNSET. Also set the default_value of DATM_CPLHIST_DIR to the users archive directory. This way if they run a case they can easily use that case for CPLHIST mode, without having to move any data around.

I'll create a PR for this that we'll discuss when I have an example out there to look at.

[ekluzek]

I talked this over with @slevis-lmwg and a little bit with @wwieder. @slevis-lmwg likes the proposed change, and would like us to try it out. If we find it causing problems, we could remove it after seeing it in practice.