Implement significant digits on output in bitc_util.py

[bas-rustenburg]

Implement using 1 significant digit on the standard error.

We can use
https://pypi.python.org/pypi/sigfig/0.1

import sigfig

x = 1.234567
e1 = 0.12
e2 = 0.012
print(sigfig.round_unc(x,e1,crop=0))
print(sigfig.round_unc(x,e2,crop=0))

which returns

1.2
1.23