[Tracker] Implement sigproc CLI tools

[pravirkr]

Implement python version of sigproc CLI tools.

This is a tracker issue that lists the remaining apps to be added.

List of APIs

IO

• chaninfo (statistics)
• chop_fil
• extract
• filedit
• filmerge
• header
• shredder
• splice (dice)
• zerodm

FRB/Pulsar

• bandpass
• best
• decimate (downsample)
• dedisperse (tree)
• dedisperse_all
• ffa
• find
• fold
• giant
• peak
• seek

Pulse Injection

• fake