Fix typos

This document provides a selection of regulatory safety objectives from CS 25 Amendment 18 (2016), the European airworthiness regulation for large aéroplanes  (FAR 25 in the US).. 
It is the entry point to carry out the safety case on muXAV using the Overarching Properties and their satisfiability criteria.
The document also provides the Accepted Means of Compliance associated to the selected CS paragraphs. It is intended to provide material supporting assessment of the OPs and criteria as a candidate AMC against CS/FAR 25.1309. and AMC 25.1309.

Updated after June workshop

The Air vehicle level Functional Hazard Analysis  (AFHA) identifies the Failure Conditions (FCs), their effects, and the severity classes of these effects. 
It is an input of many activities pertaining to the safety assessment process. In particular, it is requested for performing the Preliminary  Air vehicle level Safety Assessment (PASA).
The Failure Conditions are also requested when evaluating satisfiability of the Overarching Properties

create directories for each layer of the development assurance process

Updated based on meeting discussion,  Anne-Perrine and Alexandre comments

This reverts commit 13d59d12.

This reverts commit 7e0b5486.

First draft of the specification at AV level (step 1)

- 1st sheet : remarks
- 2nd sheet :  interfaces issues