Merge with emg3d

[prisae]

Currently emg3d-gallery is not included in emg3d as it takes a long time to run and a lot of memory.

It might still be desirable to have it all in one repo though, and there are different possibilities to achieve it.

• Have two 'docs' in emg3d, e.g, create a docs-gallery folder for this; needs adjustment of deploy.sh.
• Move away from readthedocs and deploy the entire doc in another way, merging the docs and the gallery.

[prisae]

Deploying the docs either via Travis or GitHub actions might solve the issues.

Also cutting down on the most demanding examples by

• making them slower;
• store data computed with external codes such as SimPEG and GemPy (just as done with the MARE2DEM result)

The most demanding examples (either runtime or RAM) are:

Test	minutes	GB
empymod: 1D VTI resistivity	11.0	2.8
MARE2DEM: 2D with tri-axial anisotropy	6.4	1.2
SimPEG: 3D with tri-axial anisotropy	8.1	10.1
Transient CSEM for a marine model	10.5	0.3
CPU and RAM usage	5.0	1.7
SEG-EAGE 3D Salt Model	2.6	2.5
GemPy-I: Simple Fault Model	1.2	3.1
GemPy-II: Perth Basin	2.1	4.7

[prisae]

The figures above are heavily outdated. The gallery builds currently in ~20 minutes, with <3 GB RAM.

Most demanding examples:

Test	sec	MB
as_vs_fd_gradient	219.54	9.7
gradient	207.98	454.7
timedomain	132.31	79.3
SEG-EAGE_3D_salt_model	117.35	2820.3
simulation	115.79	257.3
1D_VTI_empymod	104.25	462.6