make fallback PPR template also accept additional parameter inserts

Two things were discovered during testing today due to how we're currently working around the CASA 6.6.1 breakpoint bug:

1. When a casa version is linked away from the normal expected /home/ssa/casa and /home/casa/packages areas the capability container can't follow the path to the recipe files. This results in the matrix returning empty strings and xml errors when attempting parameter inserts.
2. The fallback PPR cannot currently be rendered with the additional specline_spw parameter needed for cal+imaging as it's being rendered outside of the casa matrix recipe retrieval.

This MR implements a stopgap correction for parameter insertion into the fallback PPR templates until we can properly design the PPR generator system. It also better handles attempting to add parameters after failure to find the PPR recipe file in the matrix itself.