Minor bug fixes

[jeswheel]

Minor bug fixes and improvements to documentation. The primary goal of these changes is to improve user experience, primarily achieved by adding helpful error messages for previously unexpected inputs.

The following changes were made:

• Added a type check for the params argument of the pfilter function in order to throw more user friendly error.
• A more user friendly error is thrown when the user specifies a parameter as both shared and unit specific in the mif2 function.
• Added a match.arg call in mif2.internal this will allow the default option for the cooling.type argument in mif2 to no longer throw an error, and mimics the behavior of the mif2 function in the pomp package.
• The pParams function no longer changes the order of the unit specific parameter names; this was causing an error because the barycentric parameter transformation requires that the parameters are adjacent to one another in the parameter vector.
• Improved documentation for panel_logmeanexp.
• More robust input checking for the pfilter function with user friendly error messages.

[jeswheel]

Note to self: missing some code coverage, and the GitHub actions appear to be broken (can't install R 4.3 for Mac).