ENH mne_experiment repr

- add **kwargs to __init__ for consistency with __repr__
- remove experiment from default __repr__ content
- in repr only show mrisubject if not the proper one
ENH data-objects:  make alternative var constructors into classmethods
ENH load.txt.tsv():  start_tag parameter
FIX data._empty_like:  raise proper error for unknown input type
ENH dataset indexing:  sequence of string returns dataset (previously list)
ENH mne_experiment:  allow setting root through e.set()
ENH data-objects:  add _effect_.count(),  factor.startswith()
ENH table.frequencies():  return dataset instead of Table
ENH plot.uv:  only escape tex syntax if mpl's text.usetex is True  (/STY)
ENH data-objects:  add sorting methods
FIX fmtxt:  .symbol()  handle df = None;  texstr handle non-dimensional arrays;

 - (/STY)
DOC
FIX test:  use ` instead of ' for trend
ENH ui:  ask_saveas() now takes defaultDir and defaultFile arguments
ENH dataset:  new save options to replace .export()
FIX dataset:

- eval X in .compress()
- remove special case for when mne.Epochs did not have len();
- add `name` argument to combine()
- __getitem__: allow index for ds.eval()
ENH mne_experiment:  inv state; load evoked

- new 'inv' state that contains complete info on inverse operation;
- improvements to load evoked methods
FIX mne_exeriment:  in .get_templates(), copy() original templates dict before modification
FIX mne_experiment:  set subjects_dir in .add_evoked_stc()
FIX? wxterm:  On startup, show dummy modal dialog, which avoids invisible windows