Crash when exporting to MRC file

[OllyK]

Issue reported by user:

### Exporting Levels ###
+ Exporting [Level 0]
+ Loading data into memory
+ Extracting data stats
+ Saving [b'Label 0'] from [Level 0]
 * Writing file [path removed]
Traceback (most recent call last):
  File "/dls_sw/apps/SuRVoS_2018/py36-build-1218/miniconda/envs/build-environment/lib/python3.6/site-packages/survos/plugins/export.py", line 145, in on_export
    self.scale.isChecked(), self.invert.isChecked())
  File "/dls_sw/apps/SuRVoS_2018/py36-build-1218/miniconda/envs/build-environment/lib/python3.6/site-packages/survos/plugins/export.py", line 214, in save_data
    self.save_mrc(outpath + '.mrc', final, owrite=owrite)
  File "/dls_sw/apps/SuRVoS_2018/py36-build-1218/miniconda/envs/build-environment/lib/python3.6/site-packages/survos/plugins/export.py", line 267, in save_mrc
    MRC(data, stats=stats).save(outpath)
  File "/dls_sw/apps/SuRVoS_2018/py36-build-1218/miniconda/envs/build-environment/lib/python3.6/site-packages/survos/lib/io.py", line 112, in __init__
    self.parse(X, stats=stats)
  File "/dls_sw/apps/SuRVoS_2018/py36-build-1218/miniconda/envs/build-environment/lib/python3.6/site-packages/survos/lib/io.py", line 254, in parse
    header = np.array(values, dtype=dt)
TypeError: a bytes-like object is required, not 'str'
Aborted (core dumped)

Related to Python3 strings?

Aha! Link: https://dls1.aha.io/features/D-22