Default experiment in qcodes

[FarBo]

This PR:

• Introduces functions to set, get and reset the active experiment. Upon loading or creating an experiment, _set_active_experiment_id will be called. get_active_experiment_id is a function to return the active exp_id in the kernel, and reset_active_experiment_id is a function to reset the active experiment to default which is the maximum exp_id in the database.
• The functions are created in qcodes.dataset.experiment_setting.
• dataset creation picks the active experiment exp_id automatically.
• We make sure to reset active experiment upon load/ create database
• test function to hit the active experiment
• Experiment notebook updated

[codecov]

Codecov Report

Merging #3214 (9457f24) into master (04b5122) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3214      +/-   ##
==========================================
+ Coverage   65.99%   66.03%   +0.03%     
==========================================
  Files         218      219       +1     
  Lines       29009    29041      +32     
==========================================
+ Hits        19145    19177      +32     
  Misses       9864     9864              

[jenshnielsen]

I think we need to spent some effort on reworking the imports. The sqlite modules should not import from the experiment container

[jenshnielsen]

@FarBo The code should now be ready please push your changes to the notebook and this should be good to go

[FarBo]

Thanks @jenshnielsen for the final work on this. I will finish the notebook today and push it, then we can merge it afterward.