[MRG] Optimize read_source_estimate function (#4107)
[MRG] Optimize read_source_estimate function (#4107)

* Optimize read_source_estimate function

Seeking to the end of the file can be a costly operation on some file
systems, especially when reading from a network mount. Since all bytes
of the file are read during this function anyway, it makes more sense to
just read everything into a buffer.

* Small tweaks + PEP8

* Use integer arithmetic

bloyl/mne-python,kingjr/mne-python,Teekuningas/mne-python,kingjr/mne-python,teonlamont/mne-python,Teekuningas/mne-python,rkmaddox/mne-python,Eric89GXL/mne-python,larsoner/mne-python,pravsripad/mne-python,kambysese/mne-python,jaeilepp/mne-python,cjayb/mne-python,rkmaddox/mne-python,olafhauk/mne-python,kingjr/mne-python,pravsripad/mne-python,drammock/mne-python,drammock/mne-python,teonlamont/mne-python,larsoner/mne-python,jaeilepp/mne-python,adykstra/mne-python,bloyl/mne-python,olafhauk/mne-python,Teekuningas/mne-python,wmvanvliet/mne-python,wmvanvliet/mne-python,cjayb/mne-python,mne-tools/mne-python,kambysese/mne-python,wmvanvliet/mne-python,mne-tools/mne-python,pravsripad/mne-python,drammock/mne-python,Eric89GXL/mne-python,adykstra/mne-python,olafhauk/mne-python,mne-tools/mne-python,larsoner/mne-python