Computation of Phi_res_crit assigns a single Pr value to all rational surfaces

[priyanshlunia]

The current implementation of slayer on the GPEC side has the following issues:

• It assigns a single value of Pr (singthresh_slayer_inpr) to all rational surfaces. It should be able to take in a list of Pr values for each corresponding rational surface (as is implemented in the SLAYER trees).
• If there is no singthresh_slayer_inpr specified, it assumes a default value of 5.0, which is not communicated. An error/warning should be thrown if it's not explicitly specified.
• Further, the "calculate slayer viscosity profile" script should populate the relevant value in the GPEC_OUTPUT tree as well.

@d-burg