snapshots, tvg, timed latches

Following functionality added:
* data snapshot functions
* tvg related functions
* timed latch functionality

Comments changed to standard adopted in fxcorrelator.py
tx/rx counter functions
Merge branch 'master' of https://github.com/ska-sa/corr2
added engine_id to engine constructor

Idea is that every engine in instrument has id allowing a user
to uniquely identify it. Fengines can have the correlator input they
are processing, Xengines the index of the frequencies they are
processing etc.
turned example into current config
configuration parsing functions

Used at various places. How configuration is done must
still be cleaned up
get_bof and host name in ping info message
renamed functions obsessively

made them <verb>_<noun> compliant
Abstraction added

feng logic for rts moved to fengine_fb_fpga as too low level to add
future fengine types
mostly renaming, hosts not checked for redundancy

host already existing allowed to be added to hosts attribute.
renaming of functions, attributes etc
FPGA based fengine producing full bandwidth
Constructor, configuration and setup logic

Works up to creation of fengines.
Requires fixing as not at correct level of abstraction/logic
sharing (e.g host creation/configuration should be done in
instrument class)