Fix time_average for instantaneous output

[uramirez8707]

Fixes #307
Adds a test to test the time_average program, without the code updates the instantaneous test will fail

[uramirez8707]

@ceblanton @rem1776

[ceblanton]

It looks like the new test is activated in the configure output and the test passes, but when i run the test interactively on ppan, or on the original test case file in the issue, it fails. Did it work interactively for you?

an010:~/git/FRE-NCtools/build10%>python3 ../t/test_time_avg.py  

 Will use input file settings for NetCDF4 deflation level
 Will use input file settings for NetCDF4 shuffle
 NETCDF ERROR in program time_average
 error getting varid for time bounds
 NetCDF: Variable not found

Program aborted. Backtrace:
#0  0x402c39 in ???
#1  0x403a63 in ???
#2  0x40a55d in ???
#3  0x40240c in ???
#4  0x2af8b7c16554 in ???
#5  0x40243c in ???
#6  0xffffffffffffffff in ???
sh: line 1: 21793 Aborted                 TAVG.exe < instantaneous.nml

[uramirez8707]

@ceblanton The test works me, are you sure there isn't an old version of frenctools in your path?

[uramirez8707]

@ceblanton I reproduced your crash when i compiled with -O3 flags. I think the problem is that the tbnds_present variable was never initialized and it ended up in this block:

FRE-NCtools/postprocessing/timavg/time_average.f90

Lines 1097 to 1100 in 3efe0e9

	istat = NF90_INQ_VARID ( ncid, trim(tbnds_name), varid(1) )
	if (istat /= NF90_NOERR) call error_handler ('error getting varid for time bounds', ncode=istat)
	istat = NF90_GET_VAR ( ncid, varid(1), tbnds )
	if (istat /= NF90_NOERR) call error_handler ('error getting data for time bounds', ncode=istat)

and complained when it couldn't find the time_bnds.

[ceblanton]

Thank you!