Fix 🩹 (almost) all warnings ⚠️ with default configuration (DIM=3, openMP+MPI, double precision, no advanced solvers, no QED, native output) (#1263)

This PR should fix almost all the compilation warnings in the default configuration (DIM=3, openMP+MPI, double precision, no advanced solvers, no QED, native output...). Or at least it does that on my system (my compiler is `g++ (Ubuntu 9.3.0-10ubuntu2) 9.3.0` ).

I have just two residual warnings:

### 1
```
[ 89%] Building CXX object CMakeFiles/WarpX.dir/Source/Parser/wp_parser.lex.cpp.o
wp_parser.lex.c:1356:17: warning: ‘void yyunput(int, char*)’ defined but not used [-Wunused-function]
```
I don't know if `yyunput(int, char*)` is there for a reason, even if it is not used. So I didn't do anything.


### 2
```
[ 96%] Building CXX object CMakeFiles/WarpX.dir/Source/Utils/CoarsenIO.cpp.o
/home/luca/Projects/warpx_dir/WarpX/Source/Particles/WarpXParticleContainer.cpp: In member function ‘void WarpXParticleContainer::AddNParticles(int, int, const ParticleReal*, const ParticleReal*, const ParticleReal*, const ParticleReal*, const ParticleReal*, const ParticleReal*, int, const ParticleReal*, int, int)’:
/home/luca/Projects/warpx_dir/WarpX/Source/Particles/WarpXParticleContainer.cpp:102:44: warning: unused parameter ‘nattr’ [-Wunused-parameter]
  102 |                                        int nattr, const ParticleReal* attr, int uniqueparticles, int id)
```
The first line of `AddNParticles` is `BL_ASSERT(nattr == 1); //! @fixme nattr is unused below: false sense of safety` . So I thought that there might be something to fix here and I didn't do anything.

1 files changed, 5 insertions, 5 deletions

diff --git a/Source/Particles/MultiParticleContainer.cpp b/Source/Particles/MultiParticleContainer.cpp
index b888bc430..d619a5de4 100644
--- a/Source/Particles/MultiParticleContainer.cpp
+++ b/Source/Particles/MultiParticleContainer.cpp
@@ -74,7 +74,7 @@ MultiParticleContainer::MultiParticleContainer (AmrCore* amr_core)
     // collision
     auto const ncollisions = collision_names.size();
     allcollisions.resize(ncollisions);
-    for (int i = 0; i < ncollisions; ++i) {
+    for (int i = 0; i < static_cast<int>(ncollisions); ++i) {
         allcollisions[i].reset
             (new CollisionType(species_names, collision_names[i]));
     }
@@ -544,7 +544,7 @@ MultiParticleContainer::doContinuousInjection () const
 void
 MultiParticleContainer::mapSpeciesProduct ()
 {
-    for (int i=0; i<species_names.size(); i++){
+    for (int i=0; i < static_cast<int>(species_names.size()); i++){
         auto& pc = allcontainers[i];
         // If species pc has ionization on, find species with name
         // pc->ionization_product_name and store its ID into
@@ -589,10 +589,10 @@ MultiParticleContainer::mapSpeciesProduct ()
 int
 MultiParticleContainer::getSpeciesID (std::string product_str) const
 {
-    int i_product;
+    int i_product = 0;
     bool found = 0;
     // Loop over species
-    for (int i=0; i<species_names.size(); i++){
+    for (int i=0; i < static_cast<int>(species_names.size()); i++){
         // If species name matches, store its ID
         // into i_product
         if (species_names[i] == product_str){
@@ -697,7 +697,7 @@ MultiParticleContainer::doCoulombCollisions ()
 
 void MultiParticleContainer::CheckIonizationProductSpecies()
 {
-    for (int i=0; i<species_names.size(); i++){
+    for (int i=0; i < static_cast<int>(species_names.size()); i++){
         if (allcontainers[i]->do_field_ionization){
             AMREX_ALWAYS_ASSERT_WITH_MESSAGE(
                 i != allcontainers[i]->ionization_product,