[WIP] Better identify dependencies for latex/pdf report functionalities

[dioph]

This introduces a new PDF_SUPPORT global boolean flag in the report module which identifies if matplotlib, pylatex and latexmk are present. This leaves both matplotlib and pylatex as optional dependencies to be installed via the PDF extras_require option. We can also ship "elegant-lite" windows installers without both these requirements.
This is related to issue #42.
I'm thinking we could rewrite the plain text report in either markdown or rST, so that at least some formatting is preserved. Any ideas?

[fmndantas]

Using markdown seems like a good idea to me. I think it is better to leave this issue opened until we commit the final report changes related to the markdown usage.

[dioph]

I'll show here a sample table with the github markdown rendering.

Bus	|V| (pu)	δ (deg)	Pg (MW)	Qg (Mvar)	Pl (MW)	Ql (Mvar)	Zl (pu)
1	1.0000	0.00	70.12	20.07	0.00	0.00	inf
2	0.9619	-2.45	0.00	0.00	0.00	0.00	inf
3	0.9192	-5.62	0.00	0.00	0.00	0.00	inf
4	0.9444	-4.15	0.00	0.00	0.00	0.00	inf
5	0.8992	-6.29	0.00	0.00	0.00	0.00	inf
6	0.9111	-5.94	0.00	0.00	0.00	0.00	inf
7	0.9014	-6.22	0.00	0.00	0.00	0.00	inf
8	0.9417	-4.23	0.00	0.00	0.00	0.00	inf
9	0.9421	-4.28	0.00	0.00	0.00	0.00	inf
10	0.9426	-4.23	0.00	0.00	0.00	0.00	inf
11	0.9367	-4.34	0.00	0.00	0.00	0.00	inf
12	0.8564	-9.97	0.00	0.00	19.00	13.50	3.15∡ 35.39
13	0.8917	-7.78	0.00	0.00	10.00	6.50	6.67∡ 33.02
14	0.8673	-9.08	0.00	0.00	15.00	11.00	4.04∡ 36.25
15	0.9197	-6.10	0.00	0.00	5.00	3.50	13.86∡ 34.99
16	0.9325	-6.13	0.00	0.00	5.00	1.50	16.66∡ 16.70
17	0.9300	-5.90	0.00	0.00	4.50	2.00	17.56∡ 23.96
18	0.8900	-8.24	0.00	0.00	10.00	7.00	6.49∡ 34.99