Check whether the extermal program exists in the path

[adityam]

Check whether the binary is present in the path using which binary or where binary.

• It is not always possible to know the name of all the binaries involved in invoking the filter command. A good approximation is to use the first word of the filtercommand and check if it is present in the path.
• For macro writers, we could provide a check key to specify a list of binaries to check.
• Another approach is to simply provide a macro \checkbinary and leave it up to the user to call that as part of filtersetup.